The Ni-containing active site of Klebsiella aerogenes urease is assembled through the concerted action of the UreD, UreE, UreF, and UreG accessory proteins. UreE functions as a metallochaperone that delivers Ni to a complex of UreD—UreF—UreG bound to urease apoprotein, with UreG serving as a GTPase during enzyme activation. The present study focuses on the role of UreF, previously proposed to act as a GTPase activating protein (GAP) of UreG. Sixteen conserved UreF surface residues that may play roles in protein:protein interactions were independently changed to Ala. When produced in the context of the entire urease gene cluster, cell-free extracts of nine site-directed mutants had less than 10% of the wild-type urease activity. Enrichment of the variant forms of UreF, as the UreE-F fusion proteins, uniformly resulted in co-purification of UreD and urease apoprotein; whereas UreG bound to only a subset of the species. Notably, reduced interaction with UreG correlated with the low activity mutants. The affected residues in UreF map to a distinct surface on the crystal structure, defining the UreG binding site. In contrast to the hypothesis that UreF is a GAP, the UreD—UreF—UreG—urease apoprotein complex containing K165A UreF exhibited significantly greater levels of GTPase activity than that containing the wild-type protein. Additional studies demonstrated the UreG GTPase activity was largely uncoupled from urease activation for the complex containing this UreF variant. Further experiments with these complexes provided evidence that UreF gates the GTPase activity of UreG to enhance the fidelity of urease metallocenter assembly, especially in the presence of the non-cognate metal Zn.

Currently, several models exist to predict the secondary structure of RNA, one of which is free energy minimization using the Nearest Neighbor Model. This model predicts the lowest free energy secondary structure from a primary sequence by summing the free energy contributions of the Watson-Crick nearest neighbor base pair combinations and any non-canonical secondary structure motif. The Nearest Neighbor Model also assumes that the free energy of the secondary structure motif is dependent solely on the identities of the nucleotides within the motif and the motif's nearest neighbors. In order to test the current assumption of the Nearest Neighbor Model that the non-nearest neighbors do not affect the stability of the motif, different stem-loop oligonucleotides were optically melted to experimentally determine their thermodynamic parameters. In each of these oligonucleotides, the hairpin loop sequence and the adjacent base pairs were held constant, while the first or second non-nearest neighbors were varied. The experimental results show that the thermodynamic contributions of the hairpin loop were dependent upon the identity of the first non-nearest neighbor, while the second non-nearest neighbor had a less obvious effect. These results were then used to create an updated model to predict the thermodynamic contributions of a hairpin loop to the overall stability of the stem-loop structure.

The mitochondrial cAMP-dependent protein kinase (PKA) is activatable in a cAMP-independent fashion. The regulatory (R) subunits of the PKA holoenzyme (R2C2), but not the catalytic (C) subunits, suffer proteolysis upon exposure of bovine heart mitochondria to digitonin, Ca2+, and a myriad of electron transport inhibitors. Selective loss of both the RI- and RII-type subunits was demonstrated via western blot analysis and activation of the C subunit was revealed by phosphorylation of a validated PKA peptide substrate. Selective proteolysis transpires in a calpain-dependent fashion as demonstrated by exposure of the R and C subunits of PKA to calpain and by attenuation of R and C subunit proteolysis in the presence of calpain inhibitor I. By contrast, exposure of mitochondria to cAMP fails to promote R subunit degradation, although it does result in enhanced C subunit catalytic activity. Treatment of mitochondria with electron transport chain inhibitors rotenone, antimycin A, sodium azide, and oligomycin, as well as an uncoupler of oxidative phosphorylation, also elicits enhanced C subunit activity. These results are consistent with the notion that signals, originating from cAMP-independent sources, elicit enhanced mitochondrial PKA activity.

Pseudomonas aeruginosa is a Gram-negative bacterium that utilizes substrate-specific outer membrane (OM) proteins for the uptake of small, water-soluble nutrients employed in the growth and function of the cell. In this paper, we present for the first time a comprehensive single-channel examination of seven members of the OM carboxylate channel K (OccK) subfamily. Recent biochemical, functional and structural characterization of the OccK proteins revealed their common features, such as a closely related, monomeric, 18-stranded β-barrel conformation with a kidney-shaped transmembrane pore and the presence of a basic ladder within the channel lumen. Here, we report that the OccK proteins exhibited fairly distinct unitary conductance values, in a much broader range than earlier expectations, which includes low (~40–100 pS) and medium (~100–380 pS) conductance. These proteins showed diverse single-channel dynamics of current gating transitions, revealing one (OccK3)-, two (OccK4, OccK5 and OccK6)- and three (OccK1, OccK2 and OccK7)-open sub-state kinetics with functionally distinct conformations. Interestingly, we discovered that anion selectivity is a conserved trait among the members of the OccK subfamily, confirming the presence of a net pool of positively charged residues within their central constriction. Moreover, these results are in accord with an increased specificity and selectivity of these protein channels for negatively charged, carboxylate-containing substrates. Our findings might ignite future functional examinations and full-atomistic computational studies for unraveling a mechanistic understanding of the passage of small molecules across the lumen of substrate-specific, β-barrel OM proteins.

We developed molecular models for the CFTR chloride channel based on the prokaryotic ABC transporter, Sav1866. Here we analyze predicted pore geometry and side-chain orientations for TMs 3, 6, 9 and 12; with particular attention to the location of the rate-limiting barrier for anion conduction. Side-chain orientations assayed by cysteine scanning were found to be from 77% to 90% in accord with model predictions. The predicted geometry of the anion conduction path was defined by a space-filling model of the pore and confirmed by visualizing the distribution of water molecules from a molecular dynamics (MD) simulation. Pore shape is that of an asymmetric hour glass, comprising a shallow outward-facing vestibule that tapers rapidly toward a narrow “bottleneck” linking the outer vestibule to a large inner cavity extending toward the cytoplasmic extent of the lipid bilayer. The junction between the outer vestibule and the bottleneck features an outward–facing rim marked by T338 in TM6 and I1131 in TM12, consistent with the observation that cysteines at both of these locations reacted with both channel-permeant and channel-impermeant, thiol-directed reagents. Conversely, cysteines substituted for S341 in TM6 or T1134 in TM12, predicted by the model to lie below the rim of the bottleneck, were found to react exclusively with channel-permeant reagents applied from the extracellular side. The predicted dimensions of the bottleneck are consistent with the demonstrated permeation of Cl− pseudohalide anions, water and urea.

Class I polyhydroxybutyrate (PHB) synthase (PhaC) from Ralstonia eutropha catalyzes the formation of PHB from (R)-3-hydroxybutyryl-CoA, ultimately resulting in the formation of insoluble granules. Previous mechanistic studies of R. eutropha PhaC, purified from Escherichia coli (PhaCEc), demonstrated that the polymer elongation rate is much faster than the initiation rate. In an effort to identify a factor(s) from the native organism that might prime the synthase and increase the rate of polymer initiation, an N-terminally Strep2-tagged phaC (Strep2-PhaCRe) was constructed and integrated into the R. eutropha genome in place of the wt-phaC. Strep2-PhaCRe was expressed and purified by affinity chromatography from R. eutropha grown in nutrient-rich TSB medium for 4 h (peak production PHB, 15% cdw) and 24 h (PHB, 2% cdw). Analysis of the purified PhaC by size exclusion chromatography, SDS-PAGE and gel permeation chromatography revealed that it unexpectedly co-purified with the phasin protein, PhaP1, and with soluble PHB (Mw 350 kDa) in a “high molecular weight” (HMW) complex and in monomeric/dimeric (M/D) forms with no associated PhaP1 or PHB. Assays to monitor PHB formation in the HMW complex showed no lag phase in CoA release, in contrast to M/D forms of PhaCRe (and PhaCEc), suggesting that PhaC in the HMW fraction has been isolated in a PHB-primed form. The presence of primed and non-primed PhaC suggests that the elongation rate for PHB formation is also faster than the initiation rate in vivo. A modified micelle model for granule genesis is proposed to accommodate the reported observations.

Focal adhesion kinase (FAK), a key regulator of cell adhesion and migration, is overexpressed in many types of cancer. The C-terminal focal adhesion targeting (FAT) domain of FAK is necessary for proper localization of FAK to focal adhesions and subsequent activation. Phosphorylation of Y926 in the FAT domain by the tyrosine kinase Src has been shown to promote metastasis and invasion in vivo by linking the FAT domain to the MAPK pathway via its interaction with Grb2. Several groups have reported that inherent conformational dynamics in the FAT domain likely regulate phosphorylation of Y926; however, what regulates these dynamics is unknown. In this paper, we demonstrate that there are two sites of in vitro Src-mediated phosphorylation in the FAT domain: Y926, which has been shown to affect FAK function in vivo, and Y1008, which has no known biological role. The phosphorylation of these two tyrosine residues is pH dependent, but this does not reflect the pH dependence of Src kinase activity. CD and NMR data indicate that the stability and conformational dynamics of the FAT domain are sensitive to changes in pH over a physiological pH range. In particular, regions of the FAT domain previously shown to regulate phosphorylation of Y926 as well as regions near Y1008 show pH-dependent dynamics on the μs-ms time scale.

Pyrroloquinoline quinone (PQQ) is a small, redox-active molecule that serves as a cofactor for several bacterial dehydrogenases, introducing pathways for carbon utilization that confer a growth advantage. Early studies had implicated a ribosomally translated peptide as the substrate for PQQ production. This study presents a sequence and structure based analysis of the components of the pqq operon. We find the necessary components for PQQ production are present in 126 prokaryotes, most of which are Gram- negative and a number of which are pathogens. A total of five gene products, PqqA, PqqB, PqqC, PqqD and PqqE, are concluded to be obligatory for PQQ production. Three of the gene products in the pqq operon, PqqB, PqqC and PqqE, are members of large protein superfamilies. By combining evolutionary conservation patterns with information from three-dimensional structures, we are able to differentiate the gene products involved in PQQ biosynthesis from those with divergent functions. The observed persistence of a conserved gene order within analyzed operons strongly suggests a role for protein/protein interactions in the course of cofactor biosynthesis. These studies propose previously unidentified roles for several of the gene products as well as possible new targets for antibiotic design and application.

Many membrane associated enzymes including those of the phospholipase C (PLC) superfamily are regulated by specific interactions with lipids. Previously we have shown that the C2 domain of PLC δ1 is required for phosphatidyl serine (PS) dependent enzyme activation, and that activation requires the presence of Ca2+. To identify the site of interaction and the role of Ca2+ in the activation mechanism, we mutagenized three highly conserved Ca2+ binding residues (Asp 653, Asp-706 and Asp-708) to Gly in the C2 domain of PLC δ1. The PS-dependent Ca2+ binding affinities of the mutant enzymes D653G, D706G and D708G were reduced by an order of magnitude and the maximal Ca2+ binding was reduced to half of that of the native enzyme. The Ca2+ dependent PS binding was also reduced in the mutant enzymes. Under basal conditions, the Ca2+ dependence and maximal hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) was not altered in the mutants. However, the Ca2+ dependent PS stimulation was severely defective. PS reduces the Km of the native enzyme almost 20 fold, but far less for the mutants. Replacing Asp-653, Asp-706 and Asp-708 simultaneously to glycine in the C2 domain of PLC δ1, leads to a complete and selective loss of the stimulation and binding by PS. These results show that D653, 706 and 708 are required for Ca2+ binding in the C2 domain and demonstrate a mechanism by which C2 domains can mediate regulation of enzyme activity by specific lipid ligands.

Eukaryotic elongation factor 2 kinase (eEF-2K) is an atypical protein kinase regulated by Ca2+ and calmodulin (CaM). Its only known substrate is eukaryotic elongation factor 2 (eEF-2), whose phosphorylation by eEF-2K impedes global protein synthesis. To date, the mechanism of eEF-2K autophosphorylation has not been fully elucidated. To investigate the mechanism of autophosphorylation, human eEF-2K was co-expressed with λ-phosphatase, and purified from bacteria in a three-step protocol using a calmodulin-affinity column. Purified eEF-2K was induced to autophosphorylate by incubation with Ca2+/CaM in the presence of MgATP. Analyzing tryptic or chymotryptic peptides by mass spectrometry monitored the autophosphorylation over 0–180 minutes. The following five major autophosphorylation sites were identified, Thr-348, Thr-353, Ser-445, Ser-474 and Ser-500. In the presence of Ca2+/CaM, robust phosphorylation of Thr-348 occurs within seconds of adding MgATP. Mutagenesis studies suggest that phosphorylation of Thr-348 is required for substrate (eEF-2 or a peptide substrate) phosphorylation, but not self-phosphorylation. Phosphorylation of Ser-500 lags behind the phosphorylation of Thr-348, and is associated with calcium-independent activity of eEF-2K. Mutation of Ser-500 to Asp, but not Ala, renders eEF-2K calcium-independent. Surprisingly, this calcium-independent activity requires the presence of calmodulin.

BtrN catalyzes the two-electron oxidation of the C3 secondary alcohol of 2-deoxy-scyllo-inosamine to the corresponding ketone, and is a member of a subclass of radical S-adenosylmethionine (SAM) enzymes called radical SAM (RS) dehydrogenases. As do all RS enzymes, BtrN contains a [4Fe–4S] cluster that delivers an electron to SAM, inducing its cleavage to the common intermediate in RS reactions, the 5’-deoxyadenosyl 5’-radical. In this work, it is shown that BtrN contains an additional [4Fe–4S] cluster, suggested to bind in contact with the substrate to facilitate loss of the second electron in the two-electron oxidation.

The recent crystal structure of two monoferric human serum transferrin (FeNhTF) molecules bound to the soluble portion of the homodimeric transferrin receptor (sTFR) has provided new details of this binding interaction which dictates iron delivery to cells. Specifically, substantial rearrangements in the homodimer interface of the sTFR occur as a result of the binding of the two FeNhTF molecules. Mutagenesis of selected residues in the sTFR highlighted in the structure was undertaken to evaluate the effect on function. Elimination of Ca2+ binding in the sTFR by mutating two of four coordinating residues ([E465A,E468A]) results in low production of an unstable and aggregated sTFR. Mutagenesis of two histidines ([H475A,H684A]) at the dimer interface had little effect on the kinetics of iron release at pH 5.6 from either lobe, reflecting the inaccessibility of this cluster to solvent. Creation of a H318A sTFR mutant allows assignment of a small pH dependent initial decrease in the fluorescent signal to His318. Removal of the four C-terminal residues of the sTFR, Asp757-Asn758-Glu759-Phe760, eliminates pH-stimulated iron release from the C-lobe of the Fe2hTF/sTFR Δ757–760 complex. The loss is accounted for by the inability of this sTFR mutant to bind and stabilize protonated hTF His349 (a pH-inducible switch) in the C-lobe of hTF. Collectively, these studies support a model in which a series of pH-induced events involving both TFR residue His318 and hTF residue His349 occurs in order to promote receptor-stimulated iron release from the C-lobe of hTF.

HIV-1 Vif is an accessory protein that induces the proteasomal degradation of the host restriction factor, apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3G (APOBEC3G). The N-terminal half of Vif binds to APOBEC3G and the C-terminal half binds to subunits of a cullin-5-based ubiquitin ligase. This Vif-directed ubiquitin ligase induces the degradation of APOBEC3G (a cytidine deaminase), and thereby protects the viral genome from mutation. A conserved PPLP motif near the C terminus of Vif is essential for Vif function and is also involved in Vif oligomerization. However, the mechanism and functional significance of Vif oligomerization is unclear. We employed analytical ultracentrifugation to examine the oligomeric properties of Vif in solution. Contrary to previous reports, we find that Vif oligomerization does not require the conserved PPLP motif. Instead, our data suggest a more complex mechanism involving interactions between the HCCH motif, BC box, and downstream residues in Vif. Mutation of residues near the PPLP motif (S165 and V166) affected the oligomeric properties of Vif and reduced the ability of Vif to bind and induce the degradation of APOBEC3G. We propose that Vif oligomerization may represent a mechanism to regulate interactions with APOBEC3G.

Despite its key role in driving cellular growth and proliferation through receptor tyrosine kinase (RTK) signaling, the Grb2-Sos1 macromolecular interaction remains poorly understood in mechanistic terms. Herein, using an array of biophysical methods, we provide evidence that although Grb2 adaptor can potentially bind to all four PXψPXR motifs — designated herein S1, S2, S3 and S4 — located within the Sos1 guanine nucleotide exchange factor, the formation of Grb2-Sos1 signaling complex occurs with a 2:1 stoichiometry. Strikingly, such bivalent binding appears to be driven by the association of Grb2 homodimer to only two out of a four potential PXψPXR motifs within Sos1 at any one time. Of particular interest is the observation that out of a possible six pairwise combinations in which S1–S4 motifs may act in concert for the docking of Grb2 homodimer through bivalent binding, only S1/S3, S1/S4, S2/S4 and S3/S4 do so, while S1/S2 and S2/S3 pairwise combinations appear to only afford monovalent binding. This salient observation implicates the role of local physical constraints in fine tuning the conformational heterogeneity of Grb2-Sos1 signaling complex. Importantly, the presence of multiple binding sites within Sos1 appears to provide a physical route for Grb2 to hop in a flip-flop manner from one site to the next through facilitated diffusion and such rapid exchange forms the basis of positive cooperativity driving the bivalent binding of Grb2 to Sos1 with high affinity. Collectively, our study sheds new light on the assembly of a key macromolecular signaling complex central to cellular machinery in health and disease.

Synthetic model peptides have proven useful for examining fundamental peptide-lipid interactions. A frequently employed peptide design consists of a hydrophobic core of Leu-Ala residues with polar or aromatic amino acids flanking each side at the interfacial positions, which serve to “anchor” a specific transmembrane orientation. For example, WALP family peptides (acetyl-GWW(LA)nLWWA-[ethanol]amide), anchored by four Trp residues, have received particular attention in both experimental and theoretical studies. A recent modification proved successful in reducing the number of Trp anchors to only one near each end of the peptide. The resulting GWALP23 (acetyl-GGALW5(LA)6LW19LAGA-[ethanol]amide) displays reduced dynamics and greater sensitivity to lipid-peptide hydrophobic mismatch than traditional WALP peptides. We have further modified GWALP23 to incorporate a single tyrosine, replacing W5 with Y5. The resulting peptide, Y5GWALP23 (acetyl-GGALY5(LA)6LW19LAGA-amide) has a single Trp residue that is sensitive to fluorescence experiments. By incorporating specific 2H and 15N labels in the core sequence of Y5GWALP23, we were able to use solid-state NMR spectroscopy to examine the peptide orientation in hydrated lipid bilayer membranes. The peptide orients well in membranes, and gives well defined 2H quadrupolar splittings and 15N/1H dipolar couplings throughout the core helical sequence between the aromatic residues. The substitution of Y5 for W5 has remarkably little influence on the tilt or dynamics of GWALP23 in bilayer membranes of the phospholipids DOPC, DMPC or DLPC. A second analogue of the peptide with one Trp and two Tyr anchors, Y4,5GWALP23, is generally less responsive to the bilayer thickness and exhibits lower apparent tilt angles with evidence of more extensive dynamics. In general, the peptide behavior with multiple Tyr anchors appears to be quite similar to the situation when multiple Trp anchors are present, as in the original WALP series of model peptides.

The EphA2 receptor plays key roles in many physiological and pathological events including cancer. The process of receptor endocytosis and the consequent degradation have lately attracted attention as possible means of overcoming the negative outcomes of EphA2 in cancer cells and decreasing tumor malignancy. A recent study indicates that Sam (Sterile Alpha Motif) domains of Odin, a member of the ANKS (Ankyrin repeat and sterile alpha motif domain-containing) family of proteins, are important to regulate EphA2 endocytosis. Odin contains two tandem Sam domains (Odin-Sam1 and Sam2).

Herein we report on the NMR solution structure of Odin-Sam1; through a variety of assays (employing NMR, SPR and ITC techniques), we clearly demonstrate that Odin-Sam1 binds to the Sam domain of EphA2 in the low micromolar range. NMR chemical shift perturbation experiments and molecular modeling studies point out that the two Sam domains interact with a head to tail topology characteristic of several Sam-Sam complexes. This binding mode is similar to that we have previously proposed for the association between the Sam domains of the lipid phosphatase Ship2 and EphA2.

This work further validates structural elements relevant for the heterotypic Sam-Sam interactions of EphA2 and provides novel insights for the design of potential therapeutic compounds that can modulate receptor endocytosis.

Prolyl-tRNA synthetases (ProRSs) have been shown to activate both cognate and some noncognate amino acids and attach them to specific tRNAPro substrates. For example, alanine, which is smaller than cognate proline, is misactivated by Escherichia coli ProRS. Mischarged Ala-tRNAPro is hydrolyzed by an editing domain (INS) that is distinct from the activation domain. It was previously shown that deletion of the INS greatly reduced cognate proline activation efficiency. In the present study, experimental and computational approaches were used to test the hypothesis that INS deletion alters the internal protein dynamics leading to reduce catalytic function. Kinetic studies with two ProRS variants, G217A and E218A, revealed decreased amino acid activation efficiency. Molecular dynamics studies showed motional coupling between the INS and protein segments containing the catalytically important proline-binding loop (PBL, residues 199–206). In particular, the complete deletion of INS, as well as mutation of G217 or E218 to alanine, exhibited significant effects on the motion of the PBL. The presence of coupled-dynamics between neighboring protein segments was also observed through in silico mutations and essential dynamics analysis. Taken together, the present study demonstrates that structural elements at the editing domain-activation domain interface participate in coupled motions that facilitate amino acid binding and catalysis by bacterial ProRSs, which may explain why truncated or defunct editing domains have been maintained in some systems, despite the lack of catalytic activity.

We have previously introduced a general kinetic approach for comparative study of processivity, thermostability, and resistance to inhibitors of DNA polymerases (Pavlov et. al., (2002) Proc. Natl. Acad. Sci. USA 99, 13510–13515). The proposed method was successfully applied to characterize hybrid DNA polymerases created by fusing catalytic DNA polymerase domains with various non-specific DNA binding domains. Here we use the developed kinetic analysis to assess basic parameters of DNA elongation by DNA polymerases and to further study the interdomain interactions in both previously constructed and new chimeric DNA polymerases. We show that connecting Helix-hairpin-Helix (HhH) domains to catalytic polymerase domains can increase thermostability, not only of DNA polymerases from extremely thermophilic species, but also of the enzyme from a faculatative thermophilic bacterium Bacillus stearothermophilus. We also demonstrate that addition of TopoV HhH domains extends efficient DNA synthesis by chimerical polymerases up to 105°C by maintaining processivity of DNA synthesis at high temperatures. We also found that reversible high-temperature structural transitions in DNA polymerases decrease the rates of binding of these enzymes to the templates. Furthermore, activation energies and pre-exponential factors of the Arrhenius equation suggest that the mechanism of electrostatic enhancement of diffusion-controlled association plays a minor role in binding templates to DNA polymerases.

Prokaryotic phosphopentomutases (PPMs) are di-Mn2+ enzymes that catalyze the interconversion of α-d-ribose 5-phosphate and α-d-ribose 1-phosphate at an active site located between two independently-folded domains. These prokaryotic PPMs belong to the alkaline phosphatase superfamily, but previous studies on Bacillus cereus PPM suggested adaptations of the conserved alkaline phosphatase catalytic cycle. Notably, B. cereus PPM engages substrate when the active site nucleophile, Thr-85, is phosphorylated. Further, the phosphoenzyme is stable throughout purification and crystallization. In contrast, alkaline phosphatase engages substrates when the active site nucleophile is dephosphorylated, and the phosphoenzyme reaction intermediate is only stably trapped in catalytically compromised enzyme. Studies were undertaken to understand the divergence of these mechanisms. Crystallographic and biochemical investigations on the PPMT85E phosphomimetic variant and the neutral corollary PPMT85Q identified that the side chain of Lys-240 changed conformation in response to active site charge, which modestly influenced affinity for the small molecule activator α-d-glucose 1,6-bisphosphate. More strikingly, the structure of unphosphorylated B. cereus PPM revealed a dramatic change in interdomain angle and a new hydrogen-bonding interaction between the side chain of Asp-156 and the active site nucleophile, Thr-85. This hydrogen-bonding interaction is predicted to align and activate Thr-85 for nucleophilic addition to α-d-glucose 1,6-bisphosphate, favoring the observed equilibrium phosphorylated state. Indeed, phosphorylation of Thr-85 is severely impaired in the PPMD156A variant even under stringent activation conditions. These results permit a proposal for activation of PPM, and explain some of the essential features that distinguish between the catalytic cycles of PPM and alkaline phosphatase.

Coupling of heterotrimeric G proteins to activated G protein-coupled receptors results in nucleotide exchange on the Gα subunit, which in turn decreases its affinity for both Gβγ and activated receptors. N-terminal myristoylation of Gα subunits aids in membrane localization of inactive G proteins. Despite the presence of the covalently attached myristoyl group, Gα proteins are highly soluble after GTP binding. This study investigated factors facilitating the solubility of the activated, myristoylated protein. In doing so, we also identified myristoylation-dependent differences in regions of Gα known to play important roles in interactions with receptors, effectors, and nucleotide binding. Amide-hydrogen deuterium exchange and site-directed fluorescence of activated proteins revealed a solvent-protected amino terminus which was enhanced by myristoylation. Furthermore, fluorescence quenching confirmed that the myristoylated amino terminus lies in close proximity to the Switch II region in the activated protein. Myristoylation also stabilized the interaction between the guanine ring and the base of the α5 helix which contacts bound nucleotide. The allosteric effects of myristoylation on protein structure, function, and localization indicate that the myristoylated amino terminus of Gαi functions as a myristoyl switch, with implications for myristoylation in the stabilization of nucleotide binding and in the spatial regulation of G protein signaling.

Parietopsin is a non-visual green-light-sensitive opsin closely related to vertebrate visual opsins, and was originally identified in lizard parietal-eye photoreceptor cells. To obtain insight into the functional diversity of opsins, we investigated by UV-visible absorption spectroscopy the molecular properties of parietopsin and its mutants exogenously expressed in cultured cells, and compared to vertebrate and invertebrate visual opsins. Our mutational analysis revealed that the counterion in parietopsin is the glutamic acid (Glu) in the second extracellular loop, corresponding to Glu181 in bovine rhodopsin. This arrangement is characteristic of invertebrate rather than vertebrate visual opsins. The photosensitivity and the molar extinction coefficient of parietopsin were also lower than those of vertebrate visual opsins, features likewise characteristic of invertebrate visual opsins. On the other hand, irradiation of parietopsin yielded meta-I, meta-II, and meta-III intermediates after batho- and lumi-intermediates, similar to vertebrate visual opsins. The pH-dependent equilibrium profile between meta-I and meta-II intermediates was, however, similar to that between acid and alkaline metarhodopsins in invertebrate visual opsins. Thus, parietopsin behaves as an “evolutionary intermediate” between invertebrate and vertebrate visual opsins.

Although the physiological role of APOBEC2 is still largely unknown, a crystal structure of a truncated variant of this protein was determined several years ago [Prochnow, C. (2007) Nature 445, 447-451]. This APOBEC2 structure had considerable impact in the HIV field since it was considered a good model for the structure of APOBEC3G, an important HIV restriction factor that abrogates HIV infectivity in the absence of the viral accessory protein Vif. The quaternary structure and the arrangement of the monomers of APOBEC2 in the crystal was taken as representative for APOBEC3G and exploited for explaining its enzymatic and anti-HIV activity. Here we show, unambiguously, that in contrast to the findings in the crystal, APOBEC2 is monomeric in solution. The NMR solution structure of full-length APOBEC2 reveals that the N-terminal tail that was removed for crystallization, resides close to strand β2, the dimer interface in the crystal structure, and shields this region of the protein from engaging in inter-molecular contacts. In addition, the presence of the N-terminal region drastically alters the aggregation propensity of APOBEC2, rendering the full-length protein highly soluble and not prone to precipitation. In summary, our results cast doubt onto all previous structure/function predictions for APOBEC3G that were based on the APOBEC2 crystal structure.