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To investigate the nanomechanics of the erythrocyte membrane we developed a hybrid model that couples the actin-spectrin network to the lipid bilayer. This model features a Fourier space Brownian dynamics model of the bilayer, a Brownian dynamics model of the actin protofilament, and a modified wormlike-chain model of the spectrin (including a cable-dynamics model to predict the oscillation in tension). This model enables us to predict the nanomechanics of single or multiple units of the protein network, the lipid bilayer, and the effect of their interactions. The present work is focused on the attitude of the actin protofilament at the equilibrium states coupled with the elevations of the lipid bilayer through their primary linkage at the suspension complex in deformations. Two different actin-spectrin junctions are considered at the junctional complex. With a point-attachment junction, large pitch angles and bifurcation of yaw angles are predicted. Thermal fluctuations at bifurcation may lead to mode-switching, which may affect the network and the physiological performance of the membrane. In contrast, with a wrap-around junction, pitch angles remain small, and the occurrence of bifurcation is greatly reduced. These simulations suggest the importance of three-dimensional molecular junctions and the lipid bilayer/protein network coupling on cell membrane mechanics.

The human hypertrophic cardiomyopathy mutation R453C is associated with a high incidence of sudden cardiac death. We have expressed the motor domain (MD) of human beta-myosin and the human ventricular essential light chain (MLC3) with the R453C mutation using a mouse muscle cell line. We have examined the binding of nucleotides to the MD and actin·MD and compared the kinetic data to our previous study of the wild type MD (Deacon et al 2012). Little change was observed in the binding of ATP or ADP to actin·MD but the two fluorescence changes accompanying ATP binding to MD in the absence of actin were slower. The fast phase, previously assigned to the conformational change accompanying tight binding of ATP to the motor, was slowed by 30%. The slow phase,assigned to the closure of switch 2 and the recovery stroke, associated with the ATP hydrolysis step, were slowed ∼ 3 fold. The R453C mutation is located in a surface loop that links the switch-2 beta strand with the long O-helix that connects to the cardiomyopathy loop in the actin binding site. R453C is therefore in a position to influence both actin binding and switch 2 closure. The surface loop and R453 are conserved only in sarcomeric myosins, suggesting a specific role for the loop in muscle. Supported by NIH grants GM29090 and GM65013 and Wellcome Trust grant 085309.