Post-translational Processing of RhoA

RhoA and related GTP-binding proteins are modified post-translationally at their carboxyl terminus to form a prenylcysteine methyl ester. The synthesis and post-translational modification of RhoA and Cdc42 were examined in the RAW264 macrophage cell line, and the effect of carboxyl methylation on protein turnover was determined. Cells were labeled with [35S]cysteine, and RhoA or Cdc42 was immunoprecipitated with specific antibodies. Both RhoA and Cdc42 were methylated rapidly in control cells, with little accumulation of unmethylated protein. Carboxyl methylation of RhoA was inhibited by incubation of cells with a carbocyclic adenosine analog, 3-deazaaristeromycin, resulting in the accumulation of unmethylated RhoA. Under these conditions, Cdc42 methylation was inhibited only partially. When methylation was inhibited, the RhoA half-life decreased from 31 to 12 h, and the Cdc42 half-life decreased from 15 to 11 h. The increased degradation of unmethylated RhoA demonstrates a novel function for carboxyl-terminal prenylcysteine carboxyl methylation in protecting RhoA and related proteins from degradation.