Troponin-I–induced tropomyosin pivoting defines thin-filament function in relaxed and active muscle

Abstract

Regulation of the crossbridge cycle that drives muscle contraction involves a reconfiguration of the troponin–tropomyosin complex on actin filaments. By comparing atomic models of troponin–tropomyosin fitted to cryo-EM structures of inhibited and Ca2+-activated thin filaments, we find that tropomyosin pivots rather than rolls or slides across actin as generally thought. We propose that pivoting can account for the Ca2+ activation that initiates muscle contraction and then relaxation influenced by troponin-I (TnI). Tropomyosin is well-known to occupy either of three meta-stable configurations on actin, regulating access of myosin motorheads to their actin-binding sites and thus the crossbridge cycle. At low Ca2+ concentrations, tropomyosin is trapped by TnI in an inhibitory B-state that sterically blocks myosin binding to actin, leading to muscle relaxation. Ca2+ binding to TnC draws TnI away from tropomyosin, while tropomyosin moves to a C-state location over actin. This partially relieves the steric inhibition and allows weak binding of myosin heads to actin, which then transition to strong actin-bound configurations, fully activating the thin filament. Nevertheless, the reconfiguration that accompanies the initial Ca2+-sensitive B-state/C-state shift in troponin–tropomyosin on actin remains uncertain and at best is described by moderate-resolution cryo-EM reconstructions. Our recent computational studies indicate that intermolecular residue-to-residue salt-bridge linkage between actin and tropomyosin is indistinguishable in B- and C-state thin filament configurations. We show here that tropomyosin can pivot about relatively fixed points on actin to accompany B-state/C-state structural transitions. We argue that at low Ca2+ concentrations C-terminal TnI domains attract tropomyosin, causing it to bend and then pivot toward the TnI, thus blocking myosin binding and contraction.

Novel Mutation Glu98Lys in Cardiac Tropomyosin Alters Its Structure and Impairs Myocardial Relaxation

“…Our MD simulation suggests that the Tpm residue Glu98 participates in the TnI-Tpm interaction, forming hydrogen bonds with the side chain of TnI residue Ser199 and a backbone atom of neighbor residue Leu198 (Figure 7). The position of Tpm residue Glu98 near TnI Ser199 and its possible involvement in the Tpm-TnI interaction were pointed out by Lehman et al [31,32]. Substituting negatively charged Glu98 for positively charged Lys would break the h-bonds between Tpm and the C-terminal part of TnI and, therefore, loosen the Tpm-TnI interaction.…”

Pivoting to a new view of tropomyosin movement

“…A small, Ca 2+ -induced shift in the cable’s position relieves this inhibition and allows the formation of myosin crossbridges, leading to muscle contraction. This positional shift is generally thought to involve tropomyosin either sliding or rolling across the surface of the actin filaments but, in this issue of JGP , Lehman and Rynkiewicz reveal that tropomyosin actually moves by pivoting around relatively fixed points on the actin subunits ( 1 ).…”

“…To understand more about the B- to C-state transition, Lehman and Rynkiewicz ( 1 ) generated atomic models of troponin and tropomyosin fitted to recent cryo-EM reconstructions of cardiac thin filaments under low and high Ca 2+ conditions ( 6 ). When viewed face on, these models clearly showed the shift in tropomyosin’s position between the low Ca 2+ B- and high Ca 2+ C-states.…”