Calmodulin is intrinsically LESS effective than troponin C in activating skeletal muscle contraction

Troponin C modulates the activation of thin filaments by rigor cross-bridges

Extraction of troponin C (TnC) from skinned muscle fibers reduces maximum Ca2+ and rigor cross-bridge (RXB)-activated tensions and reduces cooperativity between neighboring regulatory units (one troponin-tropomyosin complex and the seven associated actins) of thin filaments. This suggests that TnC has a determining role in RXB, as well as in Ca(2+)-dependent activation processes. To investigate this possibility further, we replaced fast TnC (fTnC) of rabbit psoas fibers with either CaM[3,4TnC] or cardiac TnC (cTnC) and compared the effects of these substitutions on Ca2+ and RXB activation of tension. CaM[3,4TnC] substitution has the same effect on Ca(2+)- and RXB-activated tensions; they are reduced 50%, and cooperativity between regulatory units is reduced 40%. cTnC substitution also reduces the maximum Ca(2+)-activated tension and cooperativity. But with RXB activation the effects on tension and cooperativity are opposite; cTnC substitution potentiates tension but reduces cooperativity. We considered whether tension potentiation could be explained by increased activation by cycling cross-bridges (CXBs), but the concerted transition formalism predicts fibers will fail to relax in high substrate and high pCa when CXBs are activator ligands. It predicts resting tension, which is not observed in either control or cTnC-substituted fibers. Rather, it appears that cTnC facilitates RXB activation of fast fibers more effectively than fTnC. The order of RXB-activated tension facilitation is cTnC > fTnC > CaM[3,4TnC] > empty TnC-binding sites. Comparison of the structures of fTnC, CaM[3,4TnC], and cTnC indicates that the critical region for this property lies in the central helix or N-terminal domain, including EF hand motifs 1 and 2.

The calmodulin-nitric oxide synthase interaction. Critical role of the calmodulin latch domain in enzyme activation

The neuronal isoform of nitric oxide synthase (nNOS) requires calmodulin for nitric oxide producing activity. Calmodulin functions as a molecular switch, allowing electron transport from the carboxyl-terminal reductase domain of nitric oxide synthase to its heme-containing amino-terminal domain. Available evidence suggests that calmodulin binds to a site between the two domains of nNOS, but it is not known how calmodulin then executes its switch function. To study the calmodulin-nNOS interaction, we created a series of chimeras between calmodulin and cardiac troponin C (cTnC, a homologue of calmodulin that does not activate nNOS). Although a few chimeras showed good ability to activate nNOS, most failed to activate. A subset of the inactive chimeras retained the ability to bind to nNOS and therefore functioned as potent competitive inhibitors of nNOS activation by calmodulin (CaM). The observed inhibition was additive with the arginine antagonists NG-monomethyl-L-arginine and 7-nitroindazole, indicating a distinct and independent mechanism of nNOS inhibition. To localize the calmodulin residues that account for impaired activation in the inhibitory CaM-cTnC chimeras, we conducted a detailed mutagenesis study, replacing CaM subdomains and individual amino acid residues with the corresponding residues from cTnC. This revealed that mutations in CaM helices 2 and 6 (its latch domain) have a disproportionate negative effect on nNOS activation. Thus, our evidence suggests that the CaM latch domain plays a critical role in its molecular switch function.