Cyclophilins belong to a group of proteins that possess peptidyl prolyl isomerase activity and catalyse the cis-trans conversion of proline peptide bonds. Cyclophilin members play important roles in protein folding and as molecular chaperones, in addition to a wellestablished role as host factors required for completion of the virus life cycle. Members of the cyclophilin family are overexpressed in a range of human malignancies including hepatocellular cancer, pancreatic cancer, non-small cell lung cancer, gastric cancer, colorectal cancer and glioblastoma multiforme, however, their precise role in tumourigenesis remains unclear. In recent years, mounting evidence supports a role for prolyl isomerisation during mammalian cell division; a process with striking similarity to plasma membrane remodelling during virus replication. Here we will summarise our current understanding of the role of cyclophilins in cancer. We will review the function of cyclophilins during mammalian cell division and during HIV-1 infection, and highlight common processes involving members of the ESCRT and Rab GTPase families. IntroductionThe interconversion of protein backbone between cis and trans, catalysed by peptidyl prolyl isomerases (PPIases), is an important event that alters protein structure and activity and regulates a wide spectrum of cellular functions in normal physiological processes. For example, isomerisation mediates the spatiotemporal control of fundamental processes including cell cycle progression, cell proliferation and cell death [1][2][3][4]. In recent years, mounting evidence implicates deregulated isomerase activity in a range of age-related pathologies including neurodegeneration, cardiovascular disease and cancer [5]. As a result, isomerases have gained significant interest as therapeutic targets in the treatment of these diseases.

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