ATPase caldesmon

Ngai, P.K. Walsh, M.P. Inhibition of smooth muscle actin-activated myosin Mg -ATPase activity by caldesmon. J. Biol. Chem., 259, 13656-13659 (1984)... 

Calponin is another polypeptide monomer (M.W. 32,000) that can inhibit actin-activated myosin ATPase activity. In contrast to CaD, CaP exerts its effect in the absence of tropomyosin and completely inhibits motility in a 2/3 ratio with actin. CaP inhibits myosin binding to actin, but does so by reducing the affinity of actin for myosin rather than competing for the same site. CaP can be phosphorylated by PKC and CaMKII, both of which reverse CaP s inhibitory activity. As with caldesmon, many questions remain. The ratio of CaP to actin actually observed in smooth muscle is in the range 1 10 to 1 16, far from the 2/3 ratio found to produce near-complete inhibition of motility. Therefore, the importance of CaP and its regulation by phosphorylation is still debatable. 

Caldesmon is a cytoplasmic protein with two isoform classes, one of which is found predominantly in smooth muscle cells and other cell types with partial myogenic differentiation. High-molecular-weight isoforms with molecular weights between 89 and 93 kD are capable of binding to actin, tropomyosin, calmodulin, myosin, and phospholipids, and they function to counteract actin-tropomyosin-activated myosin adenosine triphosphatase (ATPase). As such, they are mediators for the inhibition of calcium-dependent smooth muscle contraction." ... 

The possible differences in on- and off-positions of tropomyosin in smooth and skeletal muscle thin filaments may relate to significant differences in enzymatic behavior of the two systems. The well-known observation that tropomyosin activates the smooth muscle actomyosin ATPase (Chacko et al., 1977) and also accelerates the motility of smooth muscle preparations in vitro (Shirinsky et al., 1992) should be taken into account when evaluating caldesmon function. Indeed, the degree of actomyosin ATPase potentiation by tropomyosin in the smooth muscle system is considerably greater than that in skeletal muscle preparations (Chacko et al., 1977 Sobieszek and Small, 1977 Chacko, 1981 Lehrer and Morris, 1984 Williams et al.,... 

Possible caldesmon-induced movement of tropomyosin away from this site or competition with tropomyosin for the site may prevent potentiation from occurring. The work of Chacko and associates (Horiuchi and Chacko, 1989 Horiuchi et al., 1991), in fact, implies that caldesmon may modulate the magnitude of tropomyosin activation, and therefore may control actomyosin ATPase by inhibiting tropomyosin potentiation. This view would explain how a single caldesmon molecule could influence the reactivity of many actin molecules along thin filaments. Such a mechanism of modulation implies that caldesmon may fine-tune contractile activity but not act as an on-off switch per se. This view also fits with results of the elegant experiments of Fay and his collaborators (Itoh et al., 1989), who showed that myosin phosphoryla-... 

Constraints imposed by caldesmon that disrupt potentiation may not block myosin docking on actin. This process of modulating ATPase could in turn lead to or permit the development of the latch-state of tension maintenance displayed by tonic smooth muscles and observed at low Ca + concentration. This phenomenon probably involves stable actin-myosin binding and is associated with low actomyosin ATPase activity (Hai and Murphy, 1988 McDaniel et al, 1990). As envisioned, such tension maintenance would be incompatible with a troponin-tropomyosin form of regulation since tropomyosin would in that case block myosin docking at low Ca + concentrations. Hence, the caldesmon-tropomyosin system may be adapted for muscles that enter a latch-state. 

At present, the exact biological function of this protein is not known. When tested for its ability to reverse caldesmon s inhibition of the actin-activated myosin ATPase activity, 12-kDa CaBP had no significant effect on the ATPase activity when the mole ratio of 12-kDa... 

When purified CaD binds to actin-tropomyosin, it inhibits actomyosin ATPase activity. CaM in the presence of Ca + can reverse caldesmon s inhibition of the actin-activated myosin ATPase. However, the affinity of CaM for CaD is rather low (Kgg = 1Q6 M i), with the result that a large molar excess of CaM ( 25-fold) at 25°C in 60 mM KCl is required to reverse caldesmon s inhibition (Pritchard and Mar-ston, 1989 Shirinsky et al, 1988). On the other hand, CaT in the presence of 0.2 mM Ca + is very effective in releasing caldesmon s inhibition at 25°C. Complete recovery in the ATPase rate is achieved when 1 mol of CaT is added per mol of CaD (Mani etal, 1992). In fact, most of the inhibition was released (—90%) by the time... 

Phosphorylation of caldesmon in vitro by CaM-kinase II (or PKC) has been shown to interfere with caldesmon binding to F-actin, and results in a reversal of the caldesmon inhibitory effect on actin-activated myosin ATPase activity (Ngai and Walsh, 1987). This has led to speculation that these kinases may be in-... 

To account for activation of arterial smooth muscle independently of LC20 phosphorylation, attention has been focused on the possible roles of the thin filament-associated regulatory proteins, caldesmon and calponin. Both proteins have been localized in the actomyosin domain of the smooth muscle cell and both have been shown to inhibit actin-activated myosin ATPase by interacting with F-actin, tropomyosin, and/or myosin (Clark et al., 1986 Takahashi et al.,... 

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). 

Chalovich JM, Cornelius P, Benson CE (1987) Caldesmon inhibits skeletal actomyosin subfragment-1 ATPase activity and the binding of myosin subfragment-1 to actin. J Biol Chem 262 57115716... 

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