Working with a silent partner

Early stages in the production of the diverse family of biologically active molecules known as terpenes depend on the coordinated activity of various prenyltransferase (PTS) enzymes. The best characterized of these operate as ‘homomeric’ complexes containing two copies of the same protein subunit. However, new work from a team led by Tao-Hsin Chang and Andrew H.-J. Wang at the Academia Sinica in Taiwan has now revealed details of the workings of a more engimatic ‘heteromeric’ PTS, C₁₀-GPP synthase from the mint plant Mentha piperita (Mp GPPS)¹.

Mp GPPS contains two copies each of a large subunit (LSU) and a small subunit (SSU). Although LSU closely resembles the subunits of homomeric PTS complexes, Chang and Wang found that it lacks catalytic activity on its own, and depends on interactions with SSU to achieve a stable, fully functional structure.

Mp GPPS can synthesize two distinct terpene precursors of varying length, but synthesizes only the shorter of the two in vivo. Based on their crystal structure data — the first ever obtained for a heteromeric PTS — as well as a series of functional experiments, the researchers formulated a ‘two-chamber’ model in which the non-catalytic SSU acts as a regulator that physically constrains the length of molecules synthesized by the catalytic LSU.