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1.  The structure of XIAP BIR2: understanding the selectivity of the BIR domains 
The high-resolution crystal structures of apo and peptide-bound XIAP BIR2 are presented and compared with BIR3 structures to understand their selectivity. This crystal system can be used to determine the structures of BIR2–inhibitor complexes.
XIAP, a member of the inhibitor of apoptosis family of proteins, is a critical regulator of apoptosis. Inhibition of the BIR domain–caspase interaction is a promising approach towards treating cancer. Previous work has been directed towards inhibiting the BIR3–caspase-9 interaction, which blocks the intrinsic apoptotic pathway; selectively inhibiting the BIR2–caspase-3 interaction would also block the extrinsic pathway. The BIR2 domain of XIAP has successfully been crystallized; peptides and small-molecule inhibitors can be soaked into these crystals, which diffract to high resolution. Here, the BIR2 apo crystal structure and the structures of five BIR2–tetrapeptide complexes are described. The structural flexibility observed on comparing these structures, along with a comparison with XIAP BIR3, affords an understanding of the structural elements that drive selectivity between BIR2 and BIR3 and which can be used to design BIR2-selective inhibitors.
doi:10.1107/S0907444913016284
PMCID: PMC3760131  PMID: 23999295
apoptosis; XIAP; BIR domains; caspases; extrinsic pathway; inhibitor of apoptosis; peptide complex; SMAC; AVPI
2.  Nanolitre-scale crystallization using acoustic liquid-transfer technology 
Acoustic droplet ejection achieves precise, tipless, non-invasive transfer of diverse aqueous solutions, enabling nanolitre-scale crystallization trials. The rapid and scalable technique demonstrated successful crystal growth with diverse targets in drop volumes as small as 20 nl.
Focused acoustic energy allows accurate and precise liquid transfer on scales from picolitre to microlitre volumes. This technology was applied in protein crystallization, successfully transferring a diverse set of proteins as well as hundreds of precipitant solutions from custom and commercial crystallization screens and achieving crystallization in drop volumes as small as 20 nl. Only higher concentrations (>50%) of 2-­methyl-2,4-pentanediol (MPD) appeared to be systematically problematic in delivery. The acoustic technology was implemented in a workflow, successfully reproducing active crystallization systems and leading to the discovery of crystallization conditions for previously uncharacterized proteins. The technology offers compelling advantages in low-nanolitre crystallization trials by providing significant reagent savings and presenting seamless scalability for those crystals that require larger volume optimization experiments using the same vapor-diffusion format.
doi:10.1107/S0907444912016617
PMCID: PMC3413209  PMID: 22868754
acoustic liquid transfer; nanolitre-scale crystallization
3.  Structural basis for the cyclophilin A binding affinity and immunosuppressive potency of E-­ISA247 (voclosporin) 
X-ray crystal structures of the cyclosporin A analogue E-ISA247 (voclosporin) and its stereoisomer Z-ISA247 bound to cyclophilin A suggest the molecular basis for the differences in their binding affinities and immunosuppressive efficacies.
E-ISA247 (voclosporin) is a cyclosporin A analogue that is in late-stage clinical development for the treatment of autoimmune diseases and the prevention of organ graft rejection. The X-­ray crystal structures of E-ISA247 and its stereoisomer Z-­ISA247 bound to cyclophilin A have been determined and their binding affinities were measured to be 15 and 61 nM, respectively, by fluorescence spectroscopy. The higher affinity of E-ISA247 can be explained by superior van der Waals contacts between its unique side chain and cyclophilin A. Comparison with the known ternary structure including calcineurin suggests that the higher immunosuppressive efficacy of E-­ISA247 relative to cyclosporin A could be a consequence of structural changes in calcineurin induced by the modified E-­ISA247 side chain.
doi:10.1107/S0907444910051905
PMCID: PMC3045272  PMID: 21245533
voclosporin; cyclophilin A; E-ISA247; Z-ISA247

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