ATPase calponin

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. 

B. Inhibition of Actomyosin ATPase and in Vitro iMotility by Calponin... 

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). 

The mode of inhibition of actomyosin ATPase by calponin differs from that of caldesmon. The effect is mainly on the Vmax with only minor effects on the Km (Nishida et al. 1990, Horiuchi and Chacko 1991). Calponin is shown to affect strongly bound myosin (EL-Mezgueldi and Marston 1996, Borovikov et al. 1996). The inhibition is not dependent on tropomyosin (Winder and Walsh 1990) and the effects of calponin are due to changes in the actin conformation (Noda et al. 1992, Borovikov et al. 1996). 

Horiuchi KY, Chacko S (1991) The mechanism for the inhibition of actin-activated ATPase of smooth muscle heavy merom sin by calponin. Biochem Biophys Res Common 176 1487-1493... 

Calponin [119] and caldesmon [120] are two thin filament associated proteins that bind to F-actin, tropomyosin and calmodulin. Interaction of 34 kDa calponin with F-actin and tropomyosin takes place in a Ca +-inde-pendent manner, whereas that with calmodulin is regulated in a Ca -de-pendent manner. The key role of calponin and caldesmon in SM is to down-regulate actomyosin ATPase activity in vitro [120,121]. Thus, they may participate in regulation of contractile performance. Despite their apparent functional similarity, sequence analysis indicates that calponin and caldesmon are not related proteins. They act by different mechanisms... 

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