Tubulin and high molecular weight microtubule-associated proteins as endogenous substrates for protein carboxymethyltransferase in brain.
The endogenous substrate for protein carboxymethyltransferase in brain was examined. Several polypeptides were methylated when brain slices were incubated with L-methionine or when subcellular fractions of brain, such as the cytosolic fraction, were incubated with S-adenosyl L-methionine. Two methyl-accepting proteins in the cytoplasm were identified as tubulin and high molecular weight microtubule-associated proteins (300 kDa), which are components of microtubules. Tubulin behaved as a 43 kDa protein in acidic polyacrylamide gel electrophoresis, but as a 55 kDa protein in SDS-polyacrylamide gel electrophoresis. The methyl moiety transferred to these proteins from L-methionine was labile at alkaline pH. The high molecular weight microtubule-associated proteins showed higher methyl-accepting activity than tubulin or ovalbumin, which was used as a standard substrate: about 20 mmol of high molecular weight microtubule-associated proteins, 2 mmol of tubulin and 10 mmol of ovalbumin were methylated per mol of each protein in 30 min under the experimental conditions used.
D000818:Animals; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D066298:In Vitro Techniques; D008297:Male; D008715:Methionine / Q000378:metabolism; D008869:Microtubule-Associated Proteins / Q000302:isolation & purification / Q000378:metabolism*; D008970:Molecular Weight; D011496:Protein Methyltransferases / Q000378:metabolism*; D011497:Protein O-Methyltransferase / Q000302:isolation & purification / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013379:Substrate Specificity; D014404:Tubulin / Q000302:isolation & purification / Q000378:metabolism*
D008869:Microtubule-Associated Proteins; D014404:Tubulin; D008715:Methionine; D011496:Protein Methyltransferases; D011497:Protein O-Methyltransferase