Biophysical Society Thematic Meeting | Canterbury 2023

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

Monday Speaker Abstracts

ARE THE DRX AND SRX STATES IN THERMAL EQUILIBRIUM. A PARADOX? Michael A Geeves 1 ; Max Mahoney-Schaefer 2 ; Kristina Kooiker 2 ; Saffie Mohran 2 ; Jeremy Freeman 2 ; Michael Regnier 2 ; Farid Moussavi-Harami 2 ; 1 University of Kent, School of Biosciences, Canterbury, United Kingdom 2 University of Washington, Division of Cardiology & Bioengineering, Seattle, WA, USA Current views of muscle regulation suggest the presence of a dual regulation system that controls access of myosin cross-bridges to actin sites via both thin and thick filament regulation systems. Evidence for the thick filament regulation comes from structural studies of an order-disorder transition of the thick filament and biochemical assays of the rate of ATP turnover by myosin heads in HMM, myosin, thick filaments myofibrils or cells. Biochemical assays have led to the definition of a super-relaxed state (SRX) which has an ATP turnover rate < 1/10 th of that of the non-activated myosin (disordered relaxed state, DRX). Both structural and biochemical assays have been interpreted in terms of an interacting head motif (IHM) in which the two heads of a myosin molecule in the M.ADP.Pi state combine to inhibit each other. The ratio of SRX/DRX can be perturbed by temperature, small molecules (mavacamten) and myosin mutations. The problem with the simple ATPase assay of the SRX is that it is very simple to do but without care can be easily misinterpreted. Here we will show some of the problems encountered in assaying the SRX with porcine cardiac HMM & S1, and lessons that can be learnt from the assays that were done on scallop myosin before the SRX was first described. We demonstrate that there is little clear evidence for an SRX state in porcine cardiac S1 or HMM, that the SRX/DRX and order disorder transitions do not appear to correspond and the SRX and DRX states cannot be in a simple thermal equilibrium. Addition of myosin effectors such as mavacamten, danicamtiv, aficamten, omecamtiv mecarbil and deoxyATP do not appear to affect the DRX/SRX ratio for porcine cardiac HMM.

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