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Publication Abstract from PubMed

BACKGROUND: Signal Regulatory Protein gamma (SIRPgamma) is a member of a closely related family of three cell surface receptors implicated in modulating immune/inflammatory responses. SIRPgamma is expressed on T lymphocytes where it appears to be involved in the integrin-independent adhesion of lymphocytes to antigen-presenting cells. Here we describe the first full length structure of the extracellular region of human SIRPgamma. RESULTS: We obtained crystals of SIRPgamma by making a complex of the protein with the Fab fragment of the anti-SIRP antibody, OX117, which also binds to SIRPalpha and SIRPbeta. We show that the epitope for FabOX117 is formed at the interface of the first and second domains of SIRPgamma and comprises residues which are conserved between all three SIRPs. The FabOX117 binding site is distinct from the region in domain 1 which interacts with CD47, the physiological ligand for both SIRPgamma and SIRPalpha but not SIRPbeta. Comparison of the three domain structures of SIRPgamma and SIRPalpha showed that these receptors can adopt different overall conformations due to the flexibility of the linker between the first two domains. SIRPgamma in complex with FabOX117 forms a dimer in the crystal. Binding to the Fab fixes the position of domain 1 relative to domains 2/3 exposing a surface which favours formation of a homotypic dimer. However, the interaction appears to be relatively weak since only monomers of SIRPgamma were observed in sedimentation velocity analytical ultracentrifugation of the protein alone. Studies of complex formation by equilibrium ultracentrifugation showed that only a 1:1 complex of SIRPgamma: FabOX117 was formed with a dissociation constant in the low micromolar range (Kd = 1.2 +/- 0.3 muM). CONCLUSION: The three-domain extracellular regions of SIRPs are structurally conserved but show conformational flexibility in the disposition of the amino terminal ligand-binding Ig domain relative to the two membrane proximal Ig domains. Binding of a cross-reactive anti-SIRP Fab fragment to SIRPgamma stabilises a conformation that favours SIRP dimer formation in the crystal structure, though this interaction does not appear sufficiently stable to be observed in solution.

Crystal structure of signal regulatory protein gamma (SIRPgamma) in complex with an antibody Fab fragment.,Nettleship JE, Ren J, Scott DJ, Rahman N, Hatherley D, Zhao Y, Stuart DI, Barclay AN, Owens RJ BMC Struct Biol. 2013 Jul 4;13:13. doi: 10.1186/1472-6807-13-13. PMID:23826770^[2]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.