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1.  Distinct functional roles for the two SLX4 ubiquitin-binding UBZ domains mutated in Fanconi anemia 
Journal of Cell Science  2014;127(13):2811-2817.
Defects in SLX4, a scaffold for DNA repair nucleases, result in Fanconi anemia (FA), due to the defective repair of inter-strand DNA crosslinks (ICLs). Some FA patients have an SLX4 deletion removing two tandem UBZ4-type ubiquitin-binding domains that are implicated in protein recruitment to sites of DNA damage. Here, we show that human SLX4 is recruited to sites of ICL induction but that the UBZ-deleted form of SLX4 in cells from FA patients is not. SLX4 recruitment does not require either the ubiquitylation of FANCD2 or the E3 ligases RNF8, RAD18 and BRCA1. We show that the first (UBZ-1) but not the second UBZ domain of SLX4 binds to ubiquitin polymers, with a preference for K63-linked chains. Furthermore, UBZ-1 is required for SLX4 recruitment to ICL sites and for efficient ICL repair in murine fibroblasts. The SLX4 UBZ-2 domain does not bind to ubiquitin in vitro or contribute to ICL repair, but it is required for the resolution of Holliday junctions in vivo. These data shed light on SLX4 recruitment, and they point to the existence of currently unidentified ubiquitylated ligands and E3 ligases that are crucial for ICL repair.
PMCID: PMC4075355  PMID: 24794496
SLX4; FANCP; Ubiquitin; Fanconi anemia; UBZ; ICL
2.  Kinase and channel activity of TRPM6 are co-ordinated by a dimerization motif and pocket interaction 
Biochemical Journal  2014;460(Pt 2):165-175.
Mutations in the gene that encodes the atypical channel-kinase TRPM6 (transient receptor potential melastatin 6) cause HSH (hypomagnesaemia with secondary hypocalcaemia), a disorder characterized by defective intestinal Mg2+ transport and impaired renal Mg2+ reabsorption. TRPM6, together with its homologue TRPM7, are unique proteins as they combine an ion channel domain with a C-terminally fused protein kinase domain. How TRPM6 channel and kinase activity are linked is unknown. Previous structural analysis revealed that TRPM7 possesses a non-catalytic dimerization motif preceding the kinase domain. This interacts with a dimerization pocket lying within the kinase domain. In the present study, we provide evidence that the dimerization motif in TRPM6 plays a critical role in regulating kinase activity as well as ion channel activity. We identify mutations within the TRPM6 dimerization motif (Leu1718 and Leu1721) or dimerization pocket (L1743A, Q1832K, A1836N, L1840A and L1919Q) that abolish dimerization and establish that these mutations inhibit protein kinase activity. We also demonstrate that kinase activity of a dimerization motif mutant can be restored by addition of a peptide encompassing the dimerization motif. Moreover, we observe that mutations that disrupt the dimerization motif and dimerization pocket interaction greatly diminish TRPM6 ion channel activity, in a manner that is independent of kinase activity. Finally, we analyse the impact on kinase activity of ten disease-causing missense mutations that lie outwith the protein kinase domain of TRPM6. This revealed that one mutation lying nearby the dimerization motif (S1754N), found previously to inhibit channel activity, abolished kinase activity. These results provide the first evidence that there is structural co-ordination between channel and kinase activity, which is mediated by the dimerization motif and pocket interaction. We discuss that modulation of this interaction could comprise a major regulatory mechanism by which TRPM6 function is controlled.
We show that TRPM6 kinase activity is linked to channel activity. This occurs through a kinase-independent mechanism involving the dimerization motif binding to a pocket within the kinase domain. A disease-causing mutation (S1754N) lying nearby the dimerization pocket inactivates kinase activity.
PMCID: PMC4019984  PMID: 24650431
dimerization motif; hypomagnesaemia; ion channel; phosphorylation; protein kinase; transient receptor potential melastatin (TRPM); E, embryonic day; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HEK, human embryonic kidney; HRP, horseradish peroxidase; HSH, hypomagnesaemia with secondary hypocalcaemia; LDS, lithium dodecyl sulfate; MBP, myelin basic protein; TBST, TBS containing Tween 20; TRPM, transient receptor potential melastatin
3.  The WNK-regulated SPAK/OSR1 kinases directly phosphorylate and inhibit the K+–Cl− co-transporters 
Biochemical Journal  2014;458(Pt 3):559-573.
Precise homoeostasis of the intracellular concentration of Cl− is achieved via the co-ordinated activities of the Cl− influx and efflux. We demonstrate that the WNK (WNK lysine-deficient protein kinase)-activated SPAK (SPS1-related proline/alanine-rich kinase)/OSR1 (oxidative stress-responsive kinase 1) known to directly phosphorylate and stimulate the N[K]CCs (Na+–K+ ion co-transporters), also promote inhibition of the KCCs (K+–Cl− co-transporters) by directly phosphorylating a recently described C-terminal threonine residue conserved in all KCC isoforms [Site-2 (Thr1048)]. First, we demonstrate that SPAK and OSR1, in the presence of the MO25 regulatory subunit, robustly phosphorylates all KCC isoforms at Site-2 in vitro. Secondly, STOCK1S-50699, a WNK pathway inhibitor, suppresses SPAK/OSR1 activation and KCC3A Site-2 phosphorylation with similar efficiency. Thirdly, in ES (embryonic stem) cells lacking SPAK/OSR1 activity, endogenous phosphorylation of KCC isoforms at Site-2 is abolished and these cells display elevated basal activity of 86Rb+ uptake that was not markedly stimulated further by hypotonic high K+ conditions, consistent with KCC3A activation. Fourthly, a tight correlation exists between SPAK/OSR1 activity and the magnitude of KCC3A Site-2 phosphorylation. Lastly, a Site-2 alanine KCC3A mutant preventing SPAK/OSR1 phosphorylation exhibits increased activity. We also observe that KCCs are directly phosphorylated by SPAK/OSR1, at a novel Site-3 (Thr5 in KCC1/KCC3 and Thr6 in KCC2/KCC4), and a previously recognized KCC3-specific residue, Site-4 (Ser96). These data demonstrate that the WNK-regulated SPAK/OSR1 kinases directly phosphorylate the N[K]CCs and KCCs, promoting their stimulation and inhibition respectively. Given these reciprocal actions with anticipated net effects of increasing Cl− influx, we propose that the targeting of WNK–SPAK/OSR1 with kinase inhibitors might be a novel potent strategy to enhance cellular Cl− extrusion, with potential implications for the therapeutic modulation of epithelial and neuronal ion transport in human disease states.
WNK-regulated SPAK/OSR1 act as direct phosphorylators and major regulators of the KCC isoforms, which explains how activation of the WNK signalling pathway can co-ordinately regulate Cl− influx and efflux by reciprocally controlling the SLC12A family N[K]CC and KCC isoforms.
PMCID: PMC3940040  PMID: 24393035
γ-aminobutyric acid (GABA); blood pressure/hypertension; ion homoeostasis; K+–Cl− co-transporter 2 (KCC2); K+–Cl− co-transporter 3 (KCC3); Na+–Cl− co-transporter (NCC); Na+–K+–2Cl− co-transporter 1 (NKCC1); protein kinase; signal transduction; CCC, cation–Cl− co-transporter; CCT, conserved C-terminal; CTD, C-terminal cytoplasmic domain; ERK1, extracellular-signal-regulated kinase 1; ES, embryonic stem; HEK, human embryonic kidney; HRP, horseradish peroxidase; KCC, K+–Cl− co-transporter; LDS, lithium dodecyl sulfate; NCC, Na+–Cl− co-transporter; N[K]CC, Na+–K+ ion co-transporter; NKCC, Na+–K+–2Cl− co-transporter; NTD, N-terminal cytoplasmic domain; OSR1, oxidative stress-responsive kinase 1; SLC12, solute carrier family 12; SPAK, SPS1-related proline/alanine-rich kinase; TTBS, Tris-buffered saline containing Tween 20; WNK, WNK lysine-deficient protein kinase; XIC, extracted ion chromatogram
4.  Cooperative Control of Holliday Junction Resolution and DNA Repair by the SLX1 and MUS81-EME1 Nucleases 
Molecular Cell  2013;52(2):221-233.
Holliday junctions (HJs) are X-shaped DNA structures that arise during homologous recombination, which must be removed to enable chromosome segregation. The SLX1 and MUS81-EME1 nucleases can both process HJs in vitro, and they bind in close proximity on the SLX4 scaffold, hinting at possible cooperation. However, the cellular roles of mammalian SLX1 are not yet known. Here, we use mouse genetics and structure function analysis to investigate SLX1 function. Disrupting the murine Slx1 and Slx4 genes revealed that they are essential for HJ resolution in mitotic cells. Moreover, SLX1 and MUS81-EME1 act together to resolve HJs in a manner that requires tethering to SLX4. We also show that SLX1, like MUS81-EME1, is required for repair of DNA interstrand crosslinks, but this role appears to be independent of HJ cleavage, at least in mouse cells. These findings shed light on HJ resolution in mammals and on maintenance of genome stability.
Graphical Abstract
•Resolution of Holliday junctions in mouse cells requires the SLX1 nuclease•SLX1 acts cooperatively with MUS81-EME1 in HJ resolution and ICL repair•Mutations in SLX4 that prevent it binding to SLX1 and MUS81-EME1 abolish HJ resolution•DNA substrates of SLX1 and MUS81-EME1 in ICL repair appear to be different from HJs
PMCID: PMC3808987  PMID: 24076219
5.  Identification of KIAA1018/FAN1, a DNA Repair Nuclease Recruited to DNA Damage by Monoubiquitinated FANCD2 
Cell  2010;142(1):65-76.
DNA interstrand crosslinks (ICLs) are highly toxic because they block the progression of replisomes. The Fanconi Anemia (FA) proteins, encoded by genes that are mutated in FA, are important for repair of ICLs. The FA core complex catalyzes the monoubiquitination of FANCD2, and this event is essential for several steps of ICL repair. However, how monoubiquitination of FANCD2 promotes ICL repair at the molecular level is unknown. Here, we describe a highly conserved protein, KIAA1018/MTMR15/FAN1, that interacts with, and is recruited to sites of DNA damage by, the monoubiquitinated form of FANCD2. FAN1 exhibits endonuclease activity toward 5′ flaps and has 5′ exonuclease activity, and these activities are mediated by an ancient VRR_nuc domain. Depletion of FAN1 from human cells causes hypersensitivity to ICLs, defects in ICL repair, and genome instability. These data at least partly explain how ubiquitination of FANCD2 promotes DNA repair.
PMCID: PMC3710700  PMID: 20603015
6.  The CUL3–KLHL3 E3 ligase complex mutated in Gordon's hypertension syndrome interacts with and ubiquitylates WNK isoforms: disease-causing mutations in KLHL3 and WNK4 disrupt interaction 
Biochemical Journal  2013;451(Pt 1):111-122.
The WNK (with no lysine kinase)–SPAK (SPS1-related proline/alanine-rich kinase)/OSR1 (oxidative stress-responsive kinase 1) signalling pathway plays an important role in controlling mammalian blood pressure by modulating the activity of ion co-transporters in the kidney. Recent studies have identified Gordon's hypertension syndrome patients with mutations in either CUL3 (Cullin-3) or the BTB protein KLHL3 (Kelch-like 3). CUL3 assembles with BTB proteins to form Cullin–RING E3 ubiquitin ligase complexes. To explore how a CUL3–KLHL3 complex might operate, we immunoprecipitated KLHL3 and found that it associated strongly with WNK isoforms and CUL3, but not with other components of the pathway [SPAK/OSR1 or NCC (Na+/Cl− co-transporter)/NKCC1 (Na+/K+/2Cl− co-transporter 1)]. Strikingly, 13 out of the 15 dominant KLHL3 disease mutations analysed inhibited binding to WNK1 or CUL3. The recombinant wild-type CUL3–KLHL3 E3 ligase complex, but not a disease-causing CUL3–KLHL3[R528H] mutant complex, ubiquitylated WNK1 in vitro. Moreover, siRNA (small interfering RNA)-mediated knockdown of CUL3 increased WNK1 protein levels and kinase activity in HeLa cells. We mapped the KLHL3 interaction site in WNK1 to a non-catalytic region (residues 479–667). Interestingly, the equivalent region in WNK4 encompasses residues that are mutated in Gordon's syndrome patients. Strikingly, we found that the Gordon's disease-causing WNK4[E562K] and WNK4[Q565E] mutations, as well as the equivalent mutation in the WNK1[479–667] fragment, abolished the ability to interact with KLHL3. These results suggest that the CUL3–KLHL3 E3 ligase complex regulates blood pressure via its ability to interact with and ubiquitylate WNK isoforms. The findings of the present study also emphasize that the missense mutations in WNK4 that cause Gordon's syndrome strongly inhibit interaction with KLHL3. This could elevate blood pressure by increasing the expression of WNK4 thereby stimulating inappropriate salt retention in the kidney by promoting activation of the NCC/NKCC2 ion co-transporters. The present study reveals how mutations that disrupt the ability of an E3 ligase to interact with and ubiquitylate a critical cellular substrate such as WNK isoforms can trigger a chronic disease such as hypertension.
PMCID: PMC3632089  PMID: 23387299
BTB domain; Cullin–RING E3 ligase (CRL); Kelch-like domain (KLHL domain); Na+/Cl− co-transporter (NCC); Na+/K+/2Cl− co-transporter 2 (NKCC2); SPS1-related proline/alanine-rich kinase/oxidative stress-responsive kinase 1 (SPAK/OSR1); ubiquitin; CUL3, Cullin-3; CRL, Cullin–RING E3 ligase; DCT, distal convoluted tubule; DTT, dithiothreitol; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFP, green fluorescent protein; GST, glutathione transferase; HEK, human embryonic kidney; HRP, horseradish peroxidase; KEAP1, Kelch-like ECH-associated protein 1; KLHL3, Kelch-like 3; LC, liquid chromatography; NCC, Na+/Cl− co-transporter; NKCC, Na+/K+/2Cl− co-transporter; NRF2, NF-E2-related factor 2; OSR1, oxidative stress-responsive kinase 1; qRT-PCR, real time quantitative reverse transcription PCR; RBX1, RING-box 1, E3 ubiquitin protein ligase; RPL13A, ribosomal protein L13a; RT, reverse transcription; rTEV, recombinant tobacco etch virus; siRNA, small interfering RNA; SPAK, SPS1-related proline/alanine-rich kinase; TAL, thick ascending limb; TTBS, Tris-buffered saline containing Tween 20; UBE1, ubiquitin-like modifier-activating enzyme 1; UBE2D3, ubiquitin-conjugating enzyme E2 D3; WNK, with no lysine kinase
7.  Structural insights into the activation of MST3 by MO25 
► Elucidation of the structure of the catalytic core of MST3 complexed with its MO25β regulatory subunit. ► Define key conserved interface residues on MO25β and MST3 that interact with one another. ► First structure of MO25β isoform to be reported. ► Findings provide greater insight into how MO25 isoforms can function as master regulators of STE20 kinase.
The MO25 scaffolding protein operates as critical regulator of a number of STE20 family protein kinases (e.g. MST and SPAK isoforms) as well as pseudokinases (e.g. STRAD isoforms that play a critical role in activating the LKB1 tumour suppressor). To better understand how MO25 interacts and stimulates the activity of STE20 protein kinases, we determined the crystal structure of MST3 catalytic domain (residues 19–289) in complex with full length MO25β. The structure reveals an intricate web of interactions between MST3 and MO25β that function to stabilise the kinase domain in a closed, active, conformation even in the absence of ATP or an ATP-mimetic inhibitor. The binding mode of MO25β is reminiscent of the mechanism by which MO25α interacts with the pseudokinase STRADα. In particular we identified interface residues Tyr223 of MO25β and Glu58 and Ile71 of MST3 that when mutated prevent activation of MST3 by MO25β. These data provide molecular understanding of the mechanism by which MO25 isoforms regulates the activity of STE20 family protein kinases.
PMCID: PMC3725419  PMID: 23296203
LB, Luria–Bertani; MST, mammalian sterile 20 (Ste20)–like kinase; MO25, mouse protein-25; PEI, polyethyleneimine; STRAD, STE20-related adapter protein; YSK1, yeast SPS/STE20-related kinase-1; Protein kinase; LKB1; Signal transduction; Protein structure and STE20
8.  PINK1 is activated by mitochondrial membrane potential depolarization and stimulates Parkin E3 ligase activity by phosphorylating Serine 65 
Open Biology  2012;2(5):120080.
Missense mutations in PTEN-induced kinase 1 (PINK1) cause autosomal-recessive inherited Parkinson's disease (PD). We have exploited our recent discovery that recombinant insect PINK1 is catalytically active to test whether PINK1 directly phosphorylates 15 proteins encoded by PD-associated genes as well as proteins reported to bind PINK1. We have discovered that insect PINK1 efficiently phosphorylates only one of these proteins, namely the E3 ligase Parkin. We have mapped the phosphorylation site to a highly conserved residue within the Ubl domain of Parkin at Ser65. We show that human PINK1 is specifically activated by mitochondrial membrane potential (Δψm) depolarization, enabling it to phosphorylate Parkin at Ser65. We further show that phosphorylation of Parkin at Ser65 leads to marked activation of its E3 ligase activity that is prevented by mutation of Ser65 or inactivation of PINK1. We provide evidence that once activated, PINK1 autophosphorylates at several residues, including Thr257, which is accompanied by an electrophoretic mobility band-shift. These results provide the first evidence that PINK1 is activated following Δψm depolarization and suggest that PINK1 directly phosphorylates and activates Parkin. Our findings indicate that monitoring phosphorylation of Parkin at Ser65 and/or PINK1 at Thr257 represent the first biomarkers for examining activity of the PINK1-Parkin signalling pathway in vivo. Our findings also suggest that small molecule activators of Parkin that mimic the effect of PINK1 phosphorylation may confer therapeutic benefit for PD.
PMCID: PMC3376738  PMID: 22724072
PINK1; Parkin; Parkinson's disease

Results 1-8 (8)