The mammalian cytoskeletal proteins beta- and gamma-actin are highly homologous, but only beta-actin is amino-terminally arginylated in vivo, which regulates its function. We examined the metabolic fate of exogenously expressed arginylated and nonarginylated actin isoforms. Arginylated gamma-actin, unlike beta-, was highly unstable and was selectively ubiquitinated and degraded in vivo. This instability was regulated by the differences in the nucleotide coding sequence between the two actin isoforms, which conferred different translation rates. gamma-actin was translated more slowly than beta-actin, and this slower processing resulted in the exposure of a normally hidden lysine residue for ubiquitination, leading to the preferential degradation of gamma-actin upon arginylation. This degradation mechanism, coupled to nucleotide coding sequence, may regulate protein arginylation in vivo.