title: Lack of NH2-terminal processing of actin from Acanthamoeba castellanii.
abstract: Acanthamoeba actin is the only actin sequenced to date that has neither an NH2-terminal Ac-Asp nor Ac-Glu residue. The protein begins with an Ac-Gly-Asp and is coded for by a gene that specifies a polypeptide beginning Met-Gly-Asp. Thus, the Acanthamoeba actin gene would appear to specify a class II actin with the usual NH2-terminal Cys replaced with a Gly. Previous studies (Rubenstein, P. A., and Martin, D. J. (1983) J. Biol. Chem. 258, 11354-11360) revealed that for class II actins the Met is probably removed early in translation and the Cys is removed post-translationally as an Ac-Cys residue. Two possibilities might explain why Acanthamoeba actin is not processed in a similar fashion. Either Ac-Gly is not a substrate for the enzyme or the enzyme is absent from the organism. To test these alternatives, Acanthamoeba actin was labeled in vivo with [35S]methionine and incubated with processing enzyme from rat liver, rabbit reticulocytes, and Dictyostelium. In no case did the processing reaction occur, indicating that Ac-Gly is not recognized by the enzyme as a substrate. Furthermore, we could not reproducibly detect the presence of a processing enzyme in Acanthamoeba. We were, however, able to show the presence of such an enzyme in Dictyostelium, the first demonstration of this activity in a lower eukaryote.
mesh_terms: D000199:Actins / Q000096:biosynthesis / Q000235:genetics* / Q000302:isolation & purification; D000595:Amino Acid Sequence; D000656:Amoeba / Q000378:metabolism*; D000818:Animals; D004023:Dictyostelium / Q000235:genetics / Q000378:metabolism; D005796:Genes; D008715:Methionine / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D013045:Species Specificity
chemical_list: D000199:Actins; D008715:Methionine