Myosin substituted

Current evidence indicates that PLN phosphorylation appears to be dominant over troponin I phosphorylation (Li, et al., 2000). The faster SR Ca uptake by phospho-rylated PLN also contributes to increased SR Ca load, which is available for subsequent release, resulting in an inotropic effect. The increased ICa by PKA activation also contributes to the inotropic effects of the (i-AR agonists. The myofilament effects of PKA appear to be almost entirely attributable to troponin I phosphorylation (vs. myosin binding protein C) because substitution of troponin I with a non-phosphorylatable troponin I abolishes myofilament effects of PKA (Kentish, et al., 2001 Pi, et al., 2002). 

Craig, R., Padron, R., and Alamo, L. (1991). Direct determination of myosin filament symmetry in scallop striated adductor muscle by rapid freezing and freeze substitution./. Mol. Biol. 220, 125-132. 

Murphy, C. T., and Spudich, J. A. (1998). Dictyostelium myosin 25-50K loop substitutions specifically affect ADP release rates. Biochemistry 37, 6738-6744. 

G-actin is very highly conserved, both across actin genes within a species and across species. Apparently, the need for so many functional binding sites in a molecule of that size leaves few options for nonlethal mutations. Among the actins sequenced from 30 widely divergent species, there were only 32 amino acid substitutions. One implication of this is that when differences in contractile properties are observed between various types of muscle, those differences must be due to the motor protein (myosin) or to the various regulatory proteins. 

Watanabe et al. (1992) sequenced chemically the LC20 of porcine aorta myosin. There was only one amino acid substitution compared with chicken gizzard and two substitutions compared with human umbilical artery smooth muscle LC20. It is noteworthy that the substitutions never take place at positions 18 and 19, the phosphorylation sites for MLCK, or at positions 1, 2, and 9, the phosphorylation sites for PKC. 

Most of the interactions between CD and actin have been mapped to subdomain 1 of actin. This is the region of actin that in the three-dimensional structure contains both the N and C termini and important myosin and troponin I binding sites (Sheterline and Sparrow, 1994). Removal of three amino acids from the C terminus of actin weakens its interaction with both CD and troponin I (Makuch et al., 1992), and reduction of the number of acidic amino acids in the N-terminal five amino acids (substitution or deletion mutants) decreases inhibition of actin-activated ATPase by CD but does not alter its binding affinity (Crosbie et al., 1994). 

An additional substrate recognition motif appears to be contained somewhere within subdomains 1 and 2 of RLC (Zhi etai, 1994). Skeletal muscle myosin RLC is a poor substrate for the smooth muscle MLCK (Table I). Substitution of glutamate at the P-3 position for an arginine improves the ability of the skeletal muscle myosin RLC to be phosphorylated by the smooth muscle kinase however, the Pmax m ratio is only 0.06 compared to 3.4 for the wild-type smooth muscle RLC. Thus, a basic residue at the P-3 position is important, but is not the sole determinant for substrate specificity. This fact is further illustrated by the poor kinetic properties for phosphorylation of a chimera RLC contain-... 

Tsukita S, Tsukita S, Usukura J, Ishikawa H (1982) Myosin filaments in smooth muscle cells of the guinea pig taenia coli a freeze-substitution study. Bur J Cell Biol 28 195-201... 

Pharmacology. Dantrolene relaxes skeletal muscle by inhibiting the release of calcium from the sarcoplasmic reticulum, thereby reducing actin-myosin contractile activity. Dantrolene can help control hyperthermia that results from excessive muscle hyperactivity, particularly when hyperthermia is caused by a defect within the muscle cell (eg, malignant hyperthermia). Dantrolene is net a substitute for other temperature-controlling measures (eg, sponging and fanning). 

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