Caltropin

FIGURE 5 Ca + binding to caltropin. The binding assay was carried out using the calcium indicator Arsenazo III (Sigma). The solid circles represent the results from the assay. The solvent system used for the assay was 25 mM HEPES buffer, pH 7.5. The solid line represents the best fit of the data. Reprinted with permission from Biochemistry (29). Copyright 1990 American Chemical Society. 

Caltropin, which is modulated by Ca2+, binds to CaD in a Ca2+-dependent manner with high affinity. It is also very potent in regulating caldesmon s inhibition in a calcium-dependent manner and could conceivably be the Ca + factor that regulates CaD in smooth muscle. 

C, Regulation of Caldesmon and Heavy Meromyosin Interaction by Caltropin... 

Current studies suggest a vital role for CaD in the regulation of smooth muscle contraction since it inhibits the actin-activated ATPase activity of myosin (Da-browska et al., 1985 Smith and Marston, 1985 Moody et al., 1985 Sobue et al., 1985) and its subfragments (Lash et al., 1986 Chalovich et al., 1987). However, defining the precise role of CaD in cells requires an understanding of its interaction with several key proteins, namely, actin, myosin, calmodulin, and caltropin. 

FIGURE 9 A troponinlike model for cooperative caldesmon regulation of the thin filament caldesmon (CaD), tropomyosin (TM), actin (A), and myosin (M). Domain 1 corresponds to the N-terminal region of caldesmon domains 3 and 4b represent C-terminal amino acid residues from 508 to 565 and from 658 to 756, respectively. Caltropin (CaT). Reprinted with permission from J.Biol. Chem. (270). Copyright 1995. The American Society for Biochemistry and Molecular Biology. 

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