PROTEIN ISOPRENYLATION IN SUSPENSION-CULTURED TOBACCO CELLS

Many mammalian and yeast proteins, including small ras-like GTP bindin g proteins, heterotrimeric G protein gamma subunits, and nuclear lamin s, have been shown to be covalently linked to isoprenoid derivatives o f mevalonic acid. Isoprenylation of these proteins is required for the ir assembly into membranes and, hence, for their biological activity. In this report, it is shown that cultured tobacco cells, when pretreat ed with an inhibitor of endogenous mevalonic acid synthesis (lovastati n), incorporate radioactivity from C-14-mevalonic acid into proteins. Most of these proteins are membrane associated, and many are similar i n mass to mammalian ras-like GTP binding proteins and nuclear lamins. Furthermore, it is shown that tobacco cell extracts catalyze the trans fer of radioactivity from H-3-farnesyl pyrophosphate and H-3-geranylge ranyl pyrophosphate to protein substrates in vitro. These studies indi cate the presence of at least two distinct prenyl:protein transferases in tobacco extracts: one that utilizes farnesyl pyrophosphate and pre ferentially modifies a substrate protein with a CAIM carboxy terminus (farnesyl:protein transferase) and one that utilizes geranylgeranyl py rophosphate and preferentially modifies a substrate protein with a CAI L carboxy terminus (geranylgeranyl:protein transferase type 1). This w ork provides a basis for future work on the role of protein isoprenyla tion in plant cell growth, signal transduction, and membrane biogenesi s.