title: Alternate pathways for removal of the class II actin initiator methionine.
abstract: Class II actin genes usually specify a polypeptide with a Met-Cys-Asp NH2 terminus, whereas the actin itself begins with an acetyl (Ac)-Asp(Glu). Previous studies with Drosophila actin showed that the first detectable intermediate is one with an Ac-Cys NH2 terminus which is subsequently cleaved in a novel reaction to expose the Asp. The initiator Met was probably removed early in translation as a free amino acid. To determine whether the class II actin initiating Met could also be removed in an acetylation-dependent manner, we translated Drosophila mRNA in a rabbit reticulocyte lysate in which protein acetylation was inhibited. After 60 min, three actin intermediates were detected, NH2-Met-Cys-Asp-actin, Ac-Met-Cys-Asp-actin, and NH2-Cys-Asp-actin. During processing in the presence of acetyl-CoA, three additional species were observed with NH2-terminal Ac-Cys-Asp, NH2-Asp, and Ac-Asp segments. In a time- and acetyl-CoA-dependent fashion, Met-Cys-Asp-actin was processed to the mature actin, presumably through an Ac-Met-Cys-Asp intermediate. Thus, two different pathways for removal of the initiator Met of class II actins, acetylation-dependent and independent, are possible. Since no class II actin intermediate containing the initiator Met is seen in vivo, although in class I actins this intermediate is observed, the most probable pathway for class II actins in vivo is the cotranslational removal of the initiator Met as a free amino acid.
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism; D000199:Actins / Q000235:genetics* / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D004330:Drosophila; D004591:Electrophoresis, Polyacrylamide Gel; D008715:Methionine / Q000378:metabolism*; D010446:Peptide Fragments / Q000032:analysis; D013820:Thermolysin / Q000378:metabolism; D014357:Trypsin / Q000378:metabolism
chemical_list: D000199:Actins; D010446:Peptide Fragments; D000105:Acetyl Coenzyme A; D008715:Methionine; D014357:Trypsin; D013820:Thermolysin