. The actin-activated Mg"-ATPase activities of Acanthamoeba myosins I are known to be maximally expressed only when a single threonine (myosin IA) or serine (myosins IB and IC) is phosphorylated by myosin I heavy chain kinase. The purified kinase is highly activated by autophosphorylation and the rate of autophosphorylation is greatly enhanced by the presence of acidic phospholipids . In this paper, we show by immunofluorescence and immunoelectron microscopy of permeabilized cells that myosin I heavy chain kinase is highly concentrated, but not exclusively, at the plasma membrane. Judged by their electrophoretic mobilities, kinase associated with purified plasma membranes may differ from the cytoplasmic kinase, possibly in the extent of its phosphorylation . Purified kinase binds to T wo general classes of the mechanoenzyme myosin have been identified in metazoan as well as in protozoan cells . One class, the conventional two-headed myosins, myosins II by the terminology proposed by Korn and Hammer (1988), are found in muscle and nonmuscle cells . The second class, one-headed myosins referred to as myosins I (Kom and Hammer, 1988), have been purified from Acanthamoeba castelland, Dictyostelium discoideum, and intestinal brush border (for review see Korn and Hammer, 1988, 1990) and structurally related proteins yet to be characterized functionally have been identified in the photoreceptor cells ofDrosophila melanogaster (Montell and Rubin, 1988). In addition, several preliminary reports suggest that myosins I and/or other novel myosins are present in brain (Espreafico, E ., R. Chaney, F. Spindola, M. Coehlo, D. Pitta, M. Mooseker, and R. Larson. 1990. J. Cell Biol . 111: 167a; Li, D., and p. D. Chandler. 1991. J. Biophys. 52: 229a), neuronal growth cones (Bahler, 1990), and white blood cells (Atkinson and Peterson, 1991) .The best characterized mechanoenzymes of the myosin I class are those from Acanthamoeba . The three Acanthamoeba isoenzymes studied thus far, myosins IA, IB, and IC (Maruta et al., 1979 ;Lynch et al., 1989), contain a single heavy chain with an -80-kD NH2-terminal domain and an -50-kD COON-terminal domain . The NH2-terminal do-0 The Rockefeller University Press, 0021-9525/
The ultrastructure of the contractile apparatus of the rat soleus muscle during the course of denervation atrophy was investigated. It was found that the ratio of thin the thick filaments increased in myofibrils of atrophying muscle fibers. Elevation of the ratio was observed as early as the second day after denervation, and became more pronounced with the progress of atrophy. Parallel measurements of the amounts of actin and myosin in the myofibrils and in the muscle protein extracts revealed a lower proportion of myosin heavy chains to actin in the fractions from denervated muscles, compared with the control values. Both the electron-microscopic observations and the biochemical evaluation of the actin content of the muscle, suggests that the elevated ratio of thin of thick filaments seen in the course of the muscle atrophy appears as the results of an earlier and more intensive disappearance of thick filaments. Thin filaments disappeared more slowly, in parallel to the decrease in muscle weight. On the basis of the results presented a mechanism of progress of "simple atrophy" of muscle in suggested.
Changes in the contractile apparatus of denervated rat soleus muscles were investigated during the course of reinnervation. As observed earlier, in the course of denervation atrophy the ratio of myosin to actin filaments decreases because myosin filaments disappear faster than actin filaments (Jakubiec-Puka et al. 1981 a). After reinnervation the amount of myosin filaments and myosin heavy chains (myosin HC) in the muscle increased during the first few days; the increment of actin content was negligible. The proportion of myosin HC to actin remained lower than normal for about 30 days. The excess of actin filaments frequently observed in the newly-formed myofibrils reflects this disproportion. The results show a lability of myosin and suggest some cytoskeletal role for actin filaments.
The noted loss of cr-actinin from the Z-line of myotibrils during post-mortem autolysis, probably following the action of calcium-activated protease, has previously been attributed to its release without degradation. This report shows that in isolated myotibrils a-actinin is proteolysed in a Ca 2+-sensitive manner presumably via the action of calcium-activated protease.a-Actinin degradation Myofibrillar protein degradation Caz+-dependent proteolysis
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