The role of troponin-I (the inhibitory subunit of troponin) in the regulation by Ca2+ of skeletal muscle contraction was investigated with resonance energy transfer and photo cross-linking techniques. The effect of Ca2+ on the proximity of troponin-I to actin in reconstituted rabbit skeletal thin filaments was determined. The distance between the cysteine residue at position 133 (Cys133) of troponin-I and Cys374 of actin increases by approximately 15 angstroms on binding of Ca2+ to troponin-C. Also, troponin-I labeled at Cys133 with benzophenone-4-maleimide could be photo cross-linked to actin in the absence of Ca2+, but not in its presence. These results suggest that troponin-I is attached to actin in the Ca2(+)-free or relaxed state of muscle, and that it detaches from actin on Ca2+ activation of contraction. Thus, troponin-I may function as a Ca2(+)-dependent molecular switch in regulation of skeletal muscle contraction.

Calponin is a thin filament-associated protein that is implicated in the regulation and maintenance of smooth muscle contraction. Molecular cloning of chicken gizzard calponin indicated the presence of two isoforms, alpha and beta, the expression of the alpha-isoform being uniformly more abundant in various smooth muscle tissues [Takahashi, K. & Nadal-Ginard, B. (1991) J. Biol. Chem. 266, 13284-13288]. For the long-range goal of understanding of the structure and function of calponin, we have started bacterial expression and site-directed mutagenesis of alpha calponin. The amino acid composition and N-terminal sequence of the recombinant alpha calponin were found to be identical to those deduced from its nucleotide sequence. Recombinant alpha calponin is capable of binding to calmodulin, troponin C, tropomyosin, and actin, and of inhibiting skeletal muscle acto-subfragment-1 ATPase activity. A mutant alpha calponin with a replacement in the putative inhibitory region (residues 146-171) has impaired ability to inhibit the acto-subfragment-1 ATPase activity, suggesting that this region of calponin may be involved in the modulation of the actin-myosin interactions.

Growth differentiation factor 11 (GDF11), a member of the transforming growth factor β (TGF-β) family, plays diverse roles in mammalian development. It is synthesized as a large, inactive precursor protein containing a prodomain, pro-GDF11, and exists as a homodimer. Activation requires two proteolytic processing steps that release the prodomains and transform latent pro-GDF11 into active mature GDF11. In studying proteolytic activation in vitro, we discovered that a 6-kDa prodomain peptide containing residues 60-114, PDP, remained associated with the mature growth factor. Whereas the full-length prodomain of GDF11 is a functional antagonist, PDP had no impact on activity. The specific activity of the GDF11/PDP complex (EC = 1 nM) in a SMAD2/3 reporter assay was identical to that of mature GDF11 alone. PDP improved the solubility of mature GDF11 at neutral pH. As the growth factor normally aggregates/precipitates at neutral pH, PDP can be used as a solubility-enhancing formulation. Expression of two engineered constructs with PDP genetically fused to the mature domain of GDF11 through a 2x or 3x G4S linker produced soluble monomeric products that could be dimerized through redox reactions. The construct with a 3x G4S linker retained 10% activity (EC = 10 nM), whereas the construct connected with a 2x G4S linker could only be activated (EC = 2 nM) by protease treatment. Complex formation with PDP represents a new strategy for stabilizing GDF11 in an active state that may translate to other members of the TGF-β family that form latent pro/mature domain complexes.

Various thio-reactive bifunctional crosslinkers as well as 5,5'-dithiobis(2-nitrobenzoate)-mediated disulfide bond formation were used to crosslink troponin-C and troponin-I, the Ca(2+)-binding and inhibitory subunits of troponin, respectively. In all cases, substantial crosslinking was obtained when the reactions were carried out in the absence of Ca2+. No disulfide crosslinking occurred if either Cys98 of TnC, or Cys133 of TnI were blocked, indicating that these thiols are involved in the crosslinking. Troponin containing the disulfide crosslink is no longer capable of regulating actomyosin ATPase activity in a Ca(2+)-dependent manner. Our results suggest that the relative movement between the Cys98 region of TnC and the Cys133 region of TnI is required for the Ca(2+)-regulatory process in skeletal muscle.

Troponin I (TnI) is the inhibitory component of the striated muscle Ca2+ regulatory protein troponin (Tn). The other two components of Tn are troponin C (TnC), the Ca2+-binding component, and troponin T (TnT), the tropomyosin-binding component. We have used limited chymotryptic digestion to probe the local conformation of TnI in the free state, the binary TnC*TnI complex, the ternary TnC*. TnI*TnT (Tn) complex, and in the reconstituted Tn*tropomyosin*F-actin filament. The digestion of TnI alone or in the TnC*TnI complex produced initially two major fragments via a cleavage of the peptide bond between Phe100 and Asp101 in the so-called inhibitory region. In the ternary Tn complex cleavage occurred at a new site between Leu140 and Lys141. In the absence of Ca2+ this was followed by digestion of the 1-140 fragment at Leu122 and Met116. In the reconstituted thin filament the same fragments as in the case of the ternary complex were produced, but the rate of digestion was slower in the absence than in the presence of Ca2+. These results indicate firstly that in both free TnI and TnI complexed with TnC there is an exposed and flexible site in the inhibitory region. Secondly, TnT affects the conformation of TnI in the inhibitory region and also in the region that contains the 140-141 bond. Thirdly, the 140-141 region of TnI is likely to interact with actin in the reconstituted thin filament when Ca2+ is absent. These findings are discussed in terms of the role of TnI in the mechanism of thin filament regulation, and in light of our previous results [Y. Luo, J.-L. Wu, J. Gergely, T. Tao, Biochemistry 36 (1997) 13449-13454] on the global conformation of TnI.