Tissue-specific biosynthesis of epsilon-N-monomethyllysine and epsilon-N-trimethyllysine in skeletal and cardiac muscle myosin: a model for the cell-free study of post-translational amino acid modifications in proteins.

As a continuation of the study on post-ribosomal amino acid modifications in myosin, the regulation of tissue-specific biosynthesis of ?-N-monomethyllysine and ?-N-trimethyllysine was investigated. While ?-N-trimethyllysine is a component of both skeletal and cardiac muscle myosins, in certain species the monomethylated amino acids occur only in myosin from skeletal muscle. The methylation of skeletal and cardiac muscle myosin with cardiac or skeletal muscle enzymes was expected to elucidate whether the tissue-specific occurrence of the ?-N-monomethyllysine is related to the structure of skeletal and cardiac myosin or to the existence of the methylating enzyme in the skeletal and cardiac muscle cells. The experimental approach is based on cell-free methylation of lysines at 24 °C, at which temperature the myosin chains remain polysome-bound. The methylated myosin was digested with trypsin and the radioactive methyl group-containing peptides were fractionated with ion-exchange chromatography. The peptide peaks with radioactivity were subjected to amino acid analyses and the radioactive methylated lysine derivatives were identified. ?-N-trimethyllysine was found in hydrolysates of all the methylated myosins, and ?N-monomethyllysine was also present in both skeletal and cardiac muscle myosin if they were incubated with skeletal muscle supernatant. Thus the experimental results agree with our earlier suggestion (Huszar &; Elzinga, 1972) that the lack of a certain methylated amino acid in cardiac muscle myosin is due to the absence of the methylating enzyme rather than to differences in the structure of cardiac versus skeletal myosin. The experimental design developed for this work should be useful to study post-translational modifications in proteins, as well as to investigate muscle and other diseases in which the post-translational processing of proteins contributes to the dys function.