The calcium sensitizer drug MCI-154 binds the structural C-terminal domain of cardiac troponin C

Abstract

The compound MCI-154 was previously shown to increase the calcium sensitivity of cardiac muscle contraction. Using solution NMR spectroscopy, we demonstrate that MCI-154 interacts with the calcium-sensing subunit of the cardiac troponin complex, cardiac troponin C (cTnC). Surprisingly, however, it binds only to the structural C-terminal domain of cTnC (cCTnC), and not to the regulatory N-terminal domain (cNTnC) that determines the calcium sensitivity of cardiac muscle.

Physiologically, cTnC is always bound to cardiac troponin I (cTnI), so we examined its interaction with MCI-154 in the presence of two soluble constructs, cTnI _1–77 and cTnI _135–209, which contain all of the segments of cTnI known to interact with cTnC. Neither the cTnC-cTnI _1–77 complex nor the cTnC-cTnI _135–209 complex binds to MCI-154. Since residues 39–60 of cTnI are known to bind tightly to the cCTnC domain to form a structured core that is invariant throughout the cardiac cycle, we conclude that MCI-154 does not bind to cTnC when it is part of the intact cardiac troponin complex. Thus, MCI-154 likely exerts its calcium sensitizing effect by interacting with a target other than cardiac troponin.

Highlights

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  MCI-154 is a small molecule calcium sensitizer in cardiac muscle.

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  The N-domain of cardiac troponin C controls calcium sensitivity in cardiac muscle.

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  MCI-154 binds weakly to the promiscuous C-terminal domain of troponin C.

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  Cardiac troponin C does not bind MCI-154 in the presence of troponin I.

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  MCI-154 does not exert its calcium sensitizing effect directly through troponin C.