Myosin molecular weight

Nonmuscle/smooth muscle myosins-Il are structurally similar to striated muscle myosin-II, but they have slower rates of ATP hydrolysis than do their striated muscle counterparts. Nonmuscle/smooth muscle myosin-II is also regulated differently than striated muscle myosin-II. Nonmuscle myosin-II is divided into the invertebrate and vertebrate branches (Cheney et al., 1993). This group is ubiquitous because it is present in most lower organisms, such as slime molds, amoeba, sea urchins, etc., and in virtually all mammalian nonmuscle cells. Smooth muscle myosin-II is also somewhat heterogeneous in that at least three separate forms of smooth muscle heavy chains, with molecular weights of 196,000, 200,000, and 204,000 have been identified (Kawamoto and Adelstein, 1987). The physiological properties of these separate myosin heavy chains are not yet known. 

Fig. 2.1. Semilogarithmic plot of molecular weight (Mr) of marker proteins vs relative mobility (Rf) of marker proteins in gels of different acrylamide concentrations %T. Proteins 1 aprotinin (6.5 kD) 2 lysozyme (14.5 kD) 3 soybean trypsin inhibitor (21.5 kD) 4 carbonic acid anhydrase (31 kD) 5 hen ovalbumin (45 kD) 6 bovine serum albumin (66 kD) 7 phosphorylase b (97.4 kD) 8 8-galactosidase (116 kD) 9 myosin (205 kD)...

In the myosin-containing part of the sarcomere, the A-band, the myosin filaments are cross-linked at the M-band by various additional proteins (Fig. 3F, I)). Details of these are given later in Section II.C. Along the myosin filaments there is also part of the titin molecule (Fig. 5B, D). Titin is the largest known protein with a molecular weight of 3 MDa. It is anchored with its C-terminus at the M-band and its N-terminus at the Z-band. Titin... 

The Ca2+ released from the endoplasmic reticulum by IP3 may perform a number of functions. The metabolically most important action of Ca2+, however, is to combine with a very ubiquitous protein called calmodulin. This protein has a molecular weight of about 17,500 and is in many ways similar to troponin. It has four Ca2+ binding sites and constitutes one of several subunits of several enzyme systems. Thus, as cellular [Ca2+] rises, the calmodulin subunit binds Ca2+. The result is that it changes its conformation to that of an a helix and thereby affects the catalytic activity of other constituent subunits. For instance, calmodulin is a component of myosin kinase, which phosphorylates one of the subunits of myosin. The structure of calmodulin is shown in Figure 16.22. 

Fig. 4. The temporal sequence of events when a resting strip of tracheal smooth muscle is activated by carbacholamine addition at 10 min. There is a transient rise in [Ca2+]c (—) followed by a transient increase in the content (—) of phosphorylated myosin light chains (MLC-P) which lead in turn to the initiation of force development (—). Increased force is sustained even though the content of MLC-P declines. Preceding the sustained phase of force maintenance, there is an increase in the phosphorylation of desmin (D-P), synemin (S-P), caldesmon (CD-P) and a number of low molecular weight cytosolic proteins (X-P). These remain phosphorylated throughout the sustained phase of the response during which there is a sustained increase in Ca2+ cycling across the plasma membrane which regulates the activity of the membrane-associated protein kinase C.

Myosin. Rabbit muscle myosin is a long, thin molecule (VI400 X 20-50 A) with a molecular weight of 5 X 10. It is composed of two heavy chains and four light chains as demonstrated by SDS-polyacrylamide disc gel electrophoresis. On tryptic digestion, myosin is split into the subunits, H-meromyosin (HMM) and L-mero-myosin (LMM). HMM is further split into S-l and S-2 subunits. While LMM is a rod of V)0% a-helical content, the a-helical content for HMM, S-l and S-2 fragments is 46%, 33% and 87%, respectively. The ATPase activity is localized in the S-l subunit (33,34). Although fish myosins appear to have the same structural profile (10,22,35-40) and similar amino acid composition as rabbit myosin (39,41,42), fish myosin is different from rabbit myosin in physicochemical properties such as solubility, viscosity and stability (10,22,35-40). 

Myosin is easily extractable from homogenized muscle at higher ionic strength. Since actin seems to be more solidly linked to the structural element, this offers a convenient means for the separation of the two components. Myosin has been studied in some detail, and a molecular weight of the order of 400,000 to 500,000 has been reported (cf. review by Perry, 1960). 

A. Lane 1, Molecular weight markers lane 2, Purified final product. Molecular mass markers are Novex SeeBlue Pre-Stained Standards and range as follows (from top to bottom) Myosin, 250-kDa BSA, 98-kDa Glutamic dehydrogenase, 64-kDa Alcohol dehydrogenase, 50-kDa Carbonic anhydrase, 36-kDa Myoglobin, 30-kDa Lysozyme, 16-kDa Aprotinin, 6-kDa Insulin B chain, 4 kDa. 

The thin filament consists of a double tranded helix of actin molecules. The filament appears to be relatively thin, as determined by electron microscopy, accounting for the name. Troponin is part of the thin filament. It consists of a polymer of actin subunits. Each subunit has a molecular weight of 42,000. Troponin occurs as a compiex with tropomyosin, another protein of the thin filament. There is one molecule of troponin for every seven actin moiecuies in the thin fiiament- The thick filament is composed of a network of myosin molecules. Myosin has a molecular weight of 460,000 It consists of two Identical polypeptide chains. Myosin is long rather than globular or spherical. Each myosin heavy chain is associated with two myosin light chains. 

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