Role of calponin in the thin filament

Calponin has a very high binding affinity for smooth muscle F-actin which is 78 fold greater than for skeletal muscle actin (Winder et al 1991). The affinity of calponin for actin is independent of the presence of tropomyosin. The maximum in vitro binding stoichiometry of calponin to smooth muscle actin is 1 mol calponin /3 mol actin (Winder et al 1991), whereas stoichiometric ratios of calponin to actin in vivo have been estimated variously to be 1 7 (Takahashi et al 1986), 1 10 (Nishida et al 1990), and 1 16 (Marston 1991). Differences in vivo and in vitro stoichiometry may reflect an uneven distribution of calponin among different compartments of actin filaments (See Section 4.2). 

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). 

---