M-line protein preparations from frog skeletal muscle: isolation and localization of an M-line protein and a 105 000 dalton polypeptide contaminant.

A 165 000 dalton polypeptide was purified and characterized from high salt extracted crude M-line protein preparations of frog skeletal muscle. It has an isoelectric point of 5.8-6.2 and an amino acid composition similar to that reported for the putative M-line protein in chicken muscle (termed component I) and rabbit muscle. Antibodies prepared against the 165 000 dalton polypeptide not not react with purified frog debranching enzyme, and indirect immunofluorescent studies localize this polypeptide wholly in the middle of the A band (presumably the M line) of frog skeletal muscle. We conclude that this 165 000 dalton polypeptide is a basic constituent of the M line in frog skeletal muscle. Crude M-line protein preparations from frog muscle invariably contained a 105 000 dalton polypeptide contaminant. In crude M-line protein preparation from chicken muscle, a polypeptide with a similar molecular mass was identified as phosphorylase b. Characterization of the 105 000 dalton polypeptide isolated from crude M-line protein preparations establishes that in frog muscle extracts this component has an amino acid composition which is different from phosphorylase b, but similar to frog alpha-actinin. Since it also exhibits immunochemical cross-reactivity with alpha-actinin and not with purified phosphorylase b, and is localized wholly at the Z line, we conclude that the 105 000 dalton polypeptide contaminant in crude M-line protein preparations from frog muscle is alpha-actinin.