A vascular smooth muscle alpha-isoactin biosynthetic intermediate in BC3H1 cells. Identification of acetylcysteine at the NH2 terminus.

A fully translated actin biosynthetic intermediate containing N-acetylcysteine at the NH2 terminus has been identified in homogenates of differentiated mouse BC3H1 cerebrovascular smooth muscle cells labeled with L-[35S]cysteine. Thermolysin digestion of the highly acidic NH2-terminal tryptic peptide of this intermediate and electrophoretic analysis of the resulting fragments indicated that the intermediate was a precursor of smooth muscle alpha- isoactin , the major isoactin species in vascular smooth muscle. Carboxypeptidase A digestion of the thermolysin cleavage product corresponding to the first eight amino acid residues of the NH2-terminal tryptic peptide demonstrated an acetylcysteine-glutamate residue at the NH2 terminus. These results imply that the gene for smooth muscle alpha- isoactin , like genes coding for skeletal and cardiac alpha- isoactins , contains a cysteine codon immediately following the initiator methionine codon. Both the methionine and cysteine residues must be removed from the NH2 terminus of the intermediate to yield the mature form of smooth muscle alpha- isoactin . The removal of the cysteine residue in vivo is not direct but apparently involves acetylation of the cysteine and subsequent post-translational cleavage of the resulting acetylcysteine. Such an acetylation-dependent pathway has been demonstrated for removal of cysteine from the NH2 terminus of Drosophila actin synthesized in a cell-free translation system ( Rubenstein , P. A., and Martin, D. J. (1983) J. Biol. Chem. 258, 11354-11360). In vivo pulse-chase experiments indicate that the smooth muscle alpha- isoactin intermediate in BC3H1 cells turns over much more slowly than nonmuscle actin intermediates previously identified in mouse L-cells.