Inhibition of Actomyosin ATPase Activity

It is abundantly clear that in the absence of tropomyosin, CD inhibits actin activation of myosin MgATPase by reducing the affinity of weak myosin complexes for actin. This has been demonstrated directly since CD decreases the affinity of S-T ADP-Pj for 

Direct competition is structurally possible since domain 4 of CD binds to sites in or near subdomain 1 of actin, which is also known to be the site where the myosin S-1 binds (Rayment et al., 1993). S-1, troponin I, and CD contacts in this subdomain have been studied by nmr spectroscopy (Levine etal., 1990 Mornet et al., 1995), which reveals that the molecular contacts of the three proteins with amino acids in actin subdomain 1 are not the same. Nevertheless, the troponin I inhibitory peptide displaced 658C from actin, indicating that 658C is located sufficiently close to actin 1-7 that it sterically blocks this site and may thus com- 

CD inhibition of actin-activated myosin ATPase at 1 1 ratio is observable only in the laboratory. Its significance as a regulatory mechanism is severely limited because the thin filaments of smooth muscle contain only 1 CD per 14 actin (Fig. 2), so that no more than 7% inhibition is possible by this mechanism. On the other hand, it has been demonstrated in numerous laboratories that CD actin ratios similar to those in vivo are potently inhibitory when tropomyosin is present. 

The key feature of the TM-dependent inhibition of actin activity by CD is that it takes place at low ratios of CD to actin (see Fig. 6). This high degree of co-operativity is critically dependent on the TM isoform. Smooth muscle TM permits maximal inhibition at a 1 14 ratio, but filaments containing skeletal muscle TM need 1 CD 7 actin for equivalent inhibition (Marston and Redwood, 1993). When CD inhibits actin-TM-activated ATPase at physiological ratios of CD actin, there is no detectable change in the binding of S-1 

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