Biophysical Society Thematic Meeting | Canterbury 2023

Towards a More Perfect Union: Multi-Scale Models of Muscle and Their Experimental Validation

Monday Speaker Abstracts

IMPROVING SMALL-MOLECULE UPTAKE USING SIMULATIONS AND DATA TROPONIN-I INDUCED TROPOMYOSIN PIVOTING DEFINES CARDIAC THIN FILAMENT STRUCTURE IN RELAXED MUSCLE. William Lehman 1 ; Balajee Ramachandran 1 ; Michael Rynkiewicz 1 ; 1 Boston University Chobanian & Avedisian School of Medicine, Department of Physiology & Biophysics, Boston, MA, USA By occupying one of three meta-stable configurations, tropomyosin regulates access of myosin motor-heads to their actin-binding sites and thus the crossbridge cycle that drives contraction. At low-calcium concentrations, tropomyosin is trapped by troponin-I in an inhibitory B-state that sterically blocks myosin-binding to actin and leads to muscle relaxation. Calcium-binding to TnC draws TnI away from tropomyosin, while tropomyosin itself moves to a C-state location over actin. This partially relieves the steric-inhibition and allows weak-binding of myosin-heads to actin. Tropomyosin is thought to oscillate about the C-state position while myosin-heads transition to strong actin-bound configurations, fully-activating the thin filament. Nevertheless, the structural reconfiguration of thin filaments that accompanies the calcium-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 formation between actin and tropomyosin is indistinguishable in B- and C-state thin filament configurations. We ask, how then is it possible for tropomyosin to reposition, let-alone slide or roll over actin between B- and C-states, as variously proposed? Using molecular modeling and molecular dynamics simulations, we show instead that tropomyosin pivots about relatively fixed points on actin, and that this pivoting accounts for the B- to C-state change in the average center-of-mass of tropomyosin on actin. We argue that at low-calcium concentration, C-terminal domains of TnI attract the tropomyosin coiled coil which bends toward TnI to block myosin-binding, while still maintaining contact with its underlying actin-binding sites. Measurement of interaction energetics suggests that muscle activation and ensuing C-terminal TnI detachment from actin-tropomyosin involves tropomyosin pivoting back to an energetically-favorable C-state orientation.

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