S-cyclophilin: New member of the cyclophilin family associated with the secretory pathway

Cyclophilin is an abundant and ubiquitous cytosolic protein that is conserved throughout evolution from man to bacteria. It is the target of the immunosuppressive drug cyclosporin A. Cyclophilin has peptidyl-prolyl cis/trans- isomerase activity, and it accelerates protein folding in vitro, suggesting that it might be involved in the folding of cytosolic proteins. We describe a novel cyclophilin-like protein, S-cyclophilin, in the chick. Analysis of S-cyclophilin cDNA revealed the presence of a signal sequence followed by an open reading frame coding for a protein very similar to cytosolic Cyclophilin, except for the presence of unique additional short amino acid segments at the N and C termini of the protein. S-Cyclophilin mRNA was abundant and present in all embryonic chick tissues tested. Cyclophilin and S-cyclophilin are coded by separate genes in the chick genome. Recombinant S-cyclophilin was expressed in insect cells by means of the baculovirus system. Pulse-chase experiments revealed that a significant fraction of newly synthesized recombinant S-cyclophilin was rapidly secreted into the culture medium. Our findings indicate that cyclophilins are associated with most if not all intra- and extracellular compartments and suggest that enzyme-assisted conformational conversions in proteins might also take place in post-endoplasmic reticulum compartments, possibly including the extracellular space.