Calponin with Other Proteins

1991) have been disputed (Wills et al., 1993), but there is general agreement that Ca + mediates the binding of Ca + receptor molecules to CP. Wills and coworkers (1993, 1994) have analyzed in parallel the binding of 

Ligand Source (irAfl Stoichiometry of ligand calponin Reference 

Inhibition of Actomyosin ATPase and in Vitro iMotility by Calponin 

The inhibitory effect of CP on actomyosin MgATPase can be partially reversed in the presence of Ca + by the Ca + binding proteins CaM, CaT, and S-100, the latter two proteins being more effective at equimolar ratios to CP (Wills et al., 1994 Fujii et al., 1994). In line with their relatively low-affinity associations with CP (Table II), complete reversal of inhibition could be achieved only with a high molar excess of each Ca + binding protein 6-fold for CaT and S-100 (Wills et al, 1994) and 10-fold for CaM (Makuch et al., 

The same effect was observed in motility assays, where restoration of movement of actin over myosin required high concentrations (10 xM) of CaM (Shirinsky et al., 1992). CP can also be phosphoiylated in vitro and this phosphorylation abolishes its inhibitory activity (Winder and Walsh, 1990c). CP phosphorylation is considered in more detail in Section VI. 

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A family of actin-binding proteins that exist in various isoforms. As with other protein isoforms or isoenzymes, the expression of the isoforms is tissue-specific. The interaction of calponin with actin inhibits the actomyosin Mg-ATRase activity. See Winder, S. and Walsh, M., Inhibition of the actinomyosin MgATRase by chicken gizzard calponin. Prog. Clin. Biol. Res. 327, 141-148, 1990 Winder, S.J., Sutherland, C., and Walsh, M.R., Biochemical and functional characterization of smooth muscle calponin, Adv. Exp. Med. Biol. 304, 37-51, 1991 Winder, S.J. and Walsh, M.R., Calponin thin filament-linked regulation of smooth muscle contraction. Cell Signal. 5,677-686,1993 el-Mezgueldi, M., Calponin, Int. J. Biochem. 

There is considerable controversy over whether calponin and caldesmon interact on the same filament in vivo. In vitro, calponin competes with caldesmon for closely spaced sites on the actin molecule and there is evidence that the two do not complex on the same filament (Makuch et al 1991, Mezgueldi et al 1992), In addition, the inhibitory effects of calponin and caldesmon on actin-activated ATPase activity appear to be unaffected by each others presence. These observations seem to support ultrastructu-ral evidence that these proteins may localize to different actin filaments or different locations on the same filament (North et al 1994a, Makuch et al 1991) (see Section 4.2.2). 

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