<p>Cyclophilin [<cite idref="PUB00017852"/>] is the major high-affinity binding protein in vertebrates for the immunosuppressive drug cyclosporin A (CSA), but is also found in other organisms. It exhibits a peptidyl-prolyl cis-trans isomerase activity (<db_xref db="EC" dbkey="5.2.1.8"/>) (PPIase or rotamase). PPIase is an enzyme that accelerates protein folding by catalysing the cis-trans isomerisation of proline imidic peptide bonds in oligopeptides [<cite idref="PUB00000320"/>]. It is probable that CSA mediates some of its effects via an forming a tight complex with cyclophilin that inhibits the phosphatase activity of calcineurin [<cite idref="PUB00008040"/>], [<cite idref="PUB00008041"/>]. Cyclophilin A is a cytosolic and highly abundant protein. The protein belongs to a family of isozymes, including cyclophilins B and C, and natural killer cell cyclophilin-related protein [<cite idref="PUB00001515"/>, <cite idref="PUB00001443"/>, <cite idref="PUB00003851"/>]. Major isoforms have been found throughout the cell, including the ER, and some are even secreted. The sequences of the different forms of cyclophilin-type PPIases are well conserved.</p><li>Note: FKBP's, a family of proteins that bind the immunosuppressive drug FK506, are also PPIases, but their sequence is not at all related to that of cyclophilin (see <db_xref db="INTERPRO" dbkey="IPR001179"/>).</li><p>This entry represents a conserved site in the central part of these enzymes.</p> Peptidyl-prolyl cis-trans isomerase, cyclophilin-type, conserved site