Cloning and characterization of cyclophilin C-associated protein: a candidate natural cellular ligand for cyclophilin C.

We report the protein purification and the cloning and characterization of a cDNA encoding the proteins that bind with high affinity to cyclophilin C (Cyp-C) in the absence of cyclosporin A. Transfection of this cDNA into COS cells directs the production of a glycoprotein of 77 kDa that binds to Cyp-C in the absence, but not the presence, of cyclosporin A. Homology comparisons reveal that this protein and gene, termed CyCAP for Cyp-C-associated protein, possess a cysteine-rich domain (scavenger receptor cysteine-rich domain) found in a variety of cell-surface molecules; the rest of the sequence is apparently specific. This result raises the possibility that Cyp-C serves as a mediator or regulator of an as-yet-unidentified signal or cellular process initiated via the Cyp-C-associated protein.