Biosynthesis of cyclotides

Book chapter postprint DOI: 10.1016/bs.abr.2015.08.005 License: Sherpa/Romeo 0065-2296 Abstract: Cyclotides are gene-encoded, ribosomally produced cyclic proteins. Their biosynthesis is a multistep process involving several enzymes for translation, disulphide isomerization, transport, cleavage and, finally, cyclization. Here, we describe this process, focusing on the cyclization step and the vacuolar processing enzyme (VPE) that performs it. Cyclotide precursor proteins contain prodomains that direct organelle targeting and contain sequences necessary for cyclization in addition to the one or more domains that make up the final cyclic product. The VPEs are a family of cysteine proteases that perform cyclization by an adaptation of their more commonly described proteolysis mechanism. VPE-mediated cyclization is dependent on the presence of short recognition sequences (within and flanking the mature cyclotide domain) which have been characterized in vivo and in vitro for the prototypical cyclotide kalata B1. Finally, we compare cyclization by VPEs to other methods of protein cyclization found in nature or developed for biotechnology. Understanding cyclotide biosynthesis is important for both fundamental science, and for broader protein cyclization applications.