Isoprenylation of rap2 proteins in platelets and human erythroleukemia cells

The covalent modification of proteins by isoprenoid derivatives of mevalonic acid was investigated in human platelets, cells that lack the ability to synthesize endogenous cholesterol, and human erythroleukemia (HEL) cells, cholesterol-producing cultured cells derived from megakaryocytes. When washed platelets or HEL cells were incubated with 3H]mevalonic acid, the radiolabel was incorporated into a distinct group of proteins with molecular masses between 21,000 and 28,000. We have identified one of these proteins as a ras-related rap2 protein based on its immunoreactivity with a polyclonal antiserum raised against purified recombinant rap2b. This anti-rap2 antiserum was used for two-dimensional immunoblotting analysis and immunoprecipitation of mevalonate-labeled rap2 from platelets and HEL cells. These results suggest that rap2 may undergo a series of carboxyl-terminal modifications similar to the p21ras proteins. In addition, it is shown that non-cholesterol-producing cells are capable of incorporating isoprenyl groups into specific proteins.