Higher-order oligomerization of a chimeric αβγ bifunctional diterpene synthase with prenyltransferase and class II cyclase activities is concentration-dependent

The unusual diterpene (C-20) synthase copalyl diphosphate synthase from Penicilliwn verruculosum (PvCPS) is the first bifunctional terpene synthase identified with both prenyltransferase and class II cyclase activities in a single polypeptide chain with alpha beta gamma domain architecture. The C-terminal prenyltransferase alpha domain generates geranylgeranyl diphosphate which is then cyclized to form copalyl diphosphate at the N-terminal beta gamma domain interface. We now demonstrate that PvCPS exists as a hexamer at high concentrations - a unique quaternary structure for known alpha beta gamma terpene synthases. Hexamer assembly is corroborated by a 2.41 angstrom-resolution crystal structure of the a domain prenyltransferase obtained from limited proteolysis of full-length PvCPS, as well as the ab initio model of full-length PvCPS derived from small-angle X-ray scattering data. Hexamerization of the prenyltransferase a domain appears to drive the hexamerization of full-length PvCPS. The PvCPS hexamer dissociates into lower-order species at lower concentrations, as evidenced by size-exclusion chromatography inline with multiangle light scattering, sedimentation velocity analytical ultracentrifugation, and native polyacrylamide gel electrophoresis experiments, suggesting that oligomerization is concentration dependent. Even so, PvCPS oligomer assembly does not affect prenyltransferase activity in vitro.