Journal
PROTEIN SCIENCE
Volume 14, Issue 5, Pages 1214-1221Publisher
WILEY
DOI: 10.1110/ps.041270805
Keywords
platelets; calmyrin; CIB; CIB1; integrin alpha(IIb)beta(3); GPIIb/IIIa; glutathione; cysteine phosphatase
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Funding
- NCRR NIH HHS [P20RR015583, P41 RR007707, RR07707, P20 RR015583] Funding Source: Medline
- NHLBI NIH HHS [R29 HL057630, R01 HL057630, HL057630] Funding Source: Medline
- NIGMS NIH HHS [T32 GM008550] Funding Source: Medline
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Calcium- and integrin-binding protein 1 (CIB1) is involved in the process of platelet aggregation by binding the cytoplasmic tail of the alpha(IIb) subunit of the platelet-specific integrin alpha(IIb)beta(3). Although poorly understood, it is widely believed that CIB1 acts as a global signaling regulator because it is expressed in many tissues that do not express integrin alpha(IIb)beta(3). We report the structure of human CIB1 to a resolution of 2.3 angstrom, crystallized as a dimer. The dimer interface includes an extensive hydrophobic patch in a crystal form with 80% solvent content. Although the dimer form of CIB1 may not be physiologically relevant, this intersubunit surface is likely to be linked to alpha(IIb) binding and to the binding of other signaling partner proteins. The C-terminal domain of CIB1 is structurally similar to other EF-hand proteins such as calmodulin and calcineurin B. Despite structural homology to the C-terminal domain, the N-terminal domain of CIB1 lacks calcium-binding sites. The structure of CIB1 revealed a complex with a molecule of glutathione in the reduced state bond to the N-terminal domain of one of the two subunits poised to interact with the free thiol of C35. Glutathione bound in this fashion suggests CIB1 may be redox regulated. Next to the bound GSH, the orientation of residues C35, H31. and S48 is suggestive of a cysteine-type protein phosphatase active site. The potential enzymatic activity of CIB1 is discussed and suggests a mechanism by which it regulates a wide variety of proteins in cells in addition to platelets.
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