A novel myosin present in cat jaw-closing muscles

Summary

Cat jaw-closing muscles (masseter, posterior and anterior temporalis) are physiologically much faster than fast-twitch muscles of the hind limb. They have a highly oxidative metabolism and contain an alkali-stable ATPase as shown by histochemical staining. These observations have been extended here using immunocytochemical, enzymatic and biochemical analyses of myosin from posterior temporalis muscles. Antisera against posterior temporalis myosin do not precipitate gastrocnemius or soleus muscle actomyosin in Ouchterlony double diffusion. Using enzyme-linked immunoadsorbant, antisera to posterior temporalis myosin do not cross-react with heavy chains from flexor hallucis longus (FHL) or soleus myosin at dilutions where the reaction with posterior temporalis heavy chain is easily detected. Myosin isolated from posterior temporalis has an ATPase activity in the presence of EDTA (K⁺-ATPase) or calcium ions, that is, more than twice the corresponding value for FHL myosin. The myosin is unusual in its high degree of lability on storage at 0° C: it loses over 25% of the K⁺-ATPase per day and aggregates readily. Although the myosin is activated over 80-fold by actin, the maximum velocity obtained by extrapolation to saturating actin concentrations is considerably below that obtained for FHL myosin. This probably reflects the marked instability of the myosin rather than its true actin-activated ATPase. An alternative method of comparing actomyosin ATPase activity is by using myofibril preparations. Using conventional procedures it has not been possible to prepare myofibrils from posterior temporalis because of the extensive network of connective tissue surrounding the fibres. More drastic techniques, for example, the mini cell disruption chamber, will disrupt the fibres, but the myofibrils so produced are very small and often highly aggregated. Analysis of the myosin by gel electrophoresis shows that the LC1 and LC2 light chains of posterior temporalis myosin can be distinguished from those of soleus and FHL myosins. Polypeptide mapping of the various heavy chains also shows that posterior temporalis heavy chains are chemically different from those of the hind-limb muscle myosins.

Fast-twitch oxidative fibres have been identified in hind limb and body muscles of the rat, cat and chicken; posterior temporalis myosin differs from these myosins also on the basis of its histochemical properties. We conclude that the myosin of jaw-closing muscles is phenotypically distinct from both fast-twitch oxidative and fast-twitch glycolytic muscles of the body and limbs, though similar ‘super-fast’ fibres may occur elsewhere than in the jaw.