Characterization of zero-length cross-links between rabbit skeletal muscle troponin C and troponin I: evidence for direct interaction between the inhibitory region of troponin I and the amino-terminal, regulatory domain of troponin C

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Abstract
Interactions between troponin C (TnC) and troponin I (TnI) play an important role in the Ca2+-dependent regulation of vertebrate striated muscle contraction. Previous attempts to elucidate the molecular details of TnC-TnI interactions, mainly involving chemically modified proteins or fragments thereof, have led to the widely accepted idea that the "inhibitory region" (residues 96-116) of TnI binds to an .alpha.-helical segment of TnC comprising residues 89-100 in the nonregulatory, COOH-terminal domain. In an attempt to identify other possible physiologically important interactions between these proteins, 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) was used to produce zero-length cross-links in the complex of rabbit skeletal muscle TnC and TnI. TnC was activated with EDC and N-hydroxysuccinimide (NHS) and then mixed with an equimolar amount of TnI [Grabarek, Z., and Gergely, J. (1.88) Biophys. J. 53, 392a]. The resulting cross-linked TnC .times. I was cleaved with cyanogen bromide, trypsin, and Staphylococcus aureua V8 protease (SAP). Cross-linked peptides were purified by reverse-phase HPLC and characterized by sequence analysis. The results indicated that residues from the regulatory Ca2+-binding site II in the NH2-terminal domain of TnC (residues 46-78) formed cross-links with TnI segments spanning residues 92-167. The most highly cross-linked residues in TnI were Lys-105 and Lys-107, located in the inhibitory region. These results yield the first evidence for an interaction between the N-terminal domain of TnC and the inhibitory region of TnI.