Proteolytic fragmentation of brain myosin and localisation of the heavy‐chain phosphorylation site

The heavy chains and the 19‐kDa and 20‐kDa light chains of bovine brain myosin can by phosphorylated. To localise the site of heavy‐chain phosphorylation, the myosin was initially subjected to digestion with chymotrypsin and papain under a variety of conditions and the fragments thus produced were identified. Irrespective of the ionic strength, i.e. whether the myosin was monomeric or filamentous, chymotryptic digestion produced two major fragments of 68 kDa and 140 kDa; the 140‐kDa fragment was further digested by papain to yield a 120‐kDa and a 23‐kDa fragment. These fragments were characterised by (a) a gel overlay technique using ¹²⁵I‐labelled light chains, which showed that the 140‐kDa and 23‐kDa polypeptides contain the light‐chainbinding sites; (b) using myosin photoaffinity labelled at the active site with [³H]UTP, which showed that the 68‐kDa fragment contained the catalytic site, and (c) electron microscopy, using rotary shadowing and negative‐staining techniques, which demonstrated that after chymotryptic digestion the myosin head remains attached to the tail whereas on papain digestion isolated heads and tails were observed. Thus the 120‐kDa polypeptide derived from the 140‐kDa fragment is the tail of the myosin, and the 68‐kDa fragment containing the catalytic site and the 23‐kDa fragment, with the light‐chain‐binding sites, form the head (S1) portion of the myosin. When [³²P]‐phosphorylated brain myosin was digested with chymotrypsin and papain it was shown that the heavy‐chain phosphorylation site is located in a 5‐kDa peptide at the C‐terminal end of the heavy chain, i.e. the end of the myosin tail. Using hydrodynamic and electron microscopic techniques, no significant effect of either light‐chain or heavy‐chain phosphorylation on the stability of brain myosin filaments was observed, even in the presence of MgATP. Brain myosin filaments appear to be more stable than those of other non‐muscle myosins. Light‐chain phosphorylation did, however, have an effect on the conformation of brain myosin, for example in the presence of MgATP non‐phosphorylated myosin molecules were induced to fold into a very compact folded state.