New views of head-to-middle and cis- head-to-tail prenyltransferases: Structure of isosesquilavandulyl diphosphate synthase.

We report the first X-ray crystallographic structure of the head-to-middle prenyltransferase, isosesquilavandulyl diphosphate synthase, involved in biosynthesis of the merochlorin class of antibiotics. The protein adopts the or cis-prenyl transferase fold but remarkably, unlike tuberculosinol adenosine synthase and other cis-prenyl transferases (e.g. cis-farnesyl, decaprenyl, undecaprenyl diphosphate synthases), the large, hydrophobic side chain does not occupy a central hydrophobic tunnel. Instead, it occupies a surface pocket oriented at 90 degrees to the hydrophobic tunnel. Product chain-length control is achieved by squeezing out the ligand from the conventional allylic S1 binding site, with proton abstraction being achieved using a diphosphate-Asn-Ser relay. The structures revise and unify our thinking as to the mechanism of action of many other prenyl transferases and may also be of use in engineering new merochlorin-class antibiotics.