Modeling the Actin.myosin ATPase Cross-Bridge Cycle for Skeletal and Cardiac Muscle Myosin Isoforms

Abstract

Modeling the complete actin.myosin ATPase cycle has always been limited by the lack of experimental data concerning key steps of the cycle, because these steps can only be defined at very low ionic strength. Here, using human β-cardiac myosin-S1, we combine published data from transient and steady-state kinetics to model a minimal eight-state ATPase cycle. The model illustrates the occupancy of each intermediate around the cycle and how the occupancy is altered by changes in actin concentration for [actin] = 1-20K. The cycle can be used to predict the maximal velocity of contraction (by motility assay or sarcomeric shortening) at different actin concentrations (which is consistent with experimental velocity data) and predict the effect of a 5 pN load on a single motor. The same exercise was repeated for human α-cardiac myosin S1 and rabbit fast skeletal muscle S1. The data illustrates how the motor domain properties can alter the ATPase cycle and hence the occupancy of the key states in the cycle. These in turn alter the predicted mechanical response of the myosin independent of other factors present in a sarcomere, such as filament stiffness and regulatory proteins. We also explore the potential of this modeling approach for the study of mutations in human β-cardiac myosin using the hypertrophic myopathy mutation R453C. Our modeling, using the transient kinetic data, predicts mechanical properties of the motor that are compatible with the single-molecule study. The modeling approach may therefore be of wide use for predicting the properties of myosin mutations.

A Mixed-Kinetic Model Describes Unloaded Velocities of Smooth, Skeletal, and Cardiac Muscle Myosin Filaments in Vitro

“…, roughly what others have estimated under similar conditions but at lower ionic strengths (29)(30)(31).…”

“…likely include an additional slower isomerization step that occurs before ADP release (31,34,35). This value is 100 s −1 (20) for SKM, which is 4.5-fold slower than the fitted k −AD mentioned above.…”

“…S2) from fitting all three curves and use the average. This gave k −AD = 190 ± 49 s ) from modeling of solution kinetics (31). Fitting Eq.…”

Myosin motor domains carrying mutations implicated in early or late onset Hypertrophic Cardiomyopathy have similar properties

“…We modeled the complete cycle using all of our kinetic data following the approach outlined in recent papers comparing different myosin isoforms and different DCM-causing mutations in MYH7 (Srbolujub M. Mijailovich et al, 2017;Ujfalusi et al, 2018). The Table 2), and ATP-economy ( Fig.…”

“…Overall, our analysis did not reveal a pattern of common defects in individual steps of the cycle, other than a few altered properties specific to the myosin subdomain where the mutation is localized. However, a pattern did emerge when the full cycle was modeled using all the data from ATPase and stopped-flow experiments (Srbolujub M. Mijailovich et al, 2017). Modeling predicted that the DCM mutations altered the steady-state motor function and the state occupancies in the minimal 8-step crossbridge cycle in a manner consistent with a loss of the ability to generate steady-state force (Ujfalusi et al 2018).…”