Transient kinetics of the interaction of actin with myosin subfragment-1 in the absence of nucleotide

FRET-based analysis of the cardiac troponin T linker region reveals the structural basis of the hypertrophic cardiomyopathy-causing Δ160E mutation

Mutations in the cardiac thin filament (TF) have highly variable effects on the regulatory function of the cardiac sarcomere. Understanding the molecular-level dysfunction elicited by TF mutations is crucial to elucidate cardiac disease mechanisms. The hypertrophic cardiomyopathy-causing cardiac troponin T (cTnT) mutation Δ160Glu (Δ160E) is located in a putative "hinge" adjacent to an unstructured linker connecting domains TNT1 and TNT2. Currently, no high-resolution structure exists for this region, limiting significantly our ability to understand its role in myofilament activation and the molecular mechanism of mutation-induced dysfunction. Previous regulated in vitro motility data have indicated mutation-induced impairment of weak actomyosin interactions. We hypothesized that cTnT-Δ160E repositions the flexible linker, altering weak actomyosin electrostatic binding and acting as a biophysical trigger for impaired contractility and the observed remodeling. Using time-resolved FRET and an all-atom TF model, here we first defined the WT structure of the cTnT-linker region and then identified Δ160E mutation-induced positional changes. Our results suggest that the WT linker runs alongside the C terminus of tropomyosin. The Δ160E-induced structural changes moved the linker closer to the tropomyosin C terminus, an effect that was more pronounced in the presence of myosin subfragment (S1) heads, supporting previous findings. Our in silico model fully supported this result, indicating a mutation-induced decrease in linker flexibility. Our findings provide a framework for understanding basic pathogenic mechanisms that drive severe clinical hypertrophic cardiomyopathy phenotypes and for identifying structural targets for intervention that can be tested in silico and in vitro.

Interaction of isolated cross-linked short actin oligomers with the skeletal muscle myosin motor domain

The cyclical interaction between F-actin and myosin in muscle cells generates contractile force. The myosin motor domain hydrolyses ATP, resulting in conformational changes that are amplified by the myosin lever arm that links the motor domain to the rod domain. Recent cryo-electron microscopic data have provided a clear picture of the myosin-ATP-F-actin complex, but structural insights into other stages of the myosin-actin interaction have been less forthcoming. To address this issue, we cross-linked F-actin subunits between Cys374 and Lys191, and separated them by gel filtration. Purified actin-dimers, -trimers and -tetramers retained the ability to polymerize and to stimulate myosin-subfragment 1 (myosin-S1) ATPase activity. To generate stable actin oligomer:myosin-S1 complexes, we blocked actin polymerization with gelsolin and Clostridium botulinum iota toxin-mediated ADP-ribosylation. After polymerization inhibition, actin-trimers and -tetramers retained the ability to stimulate the myosin-S1-ATPase, whereas the actin-dimer showed very little ATPase stimulation. We then analysed the stoichiometry and binding affinity of myosin-S1 to actin oligomers. Actin-trimers and -tetramers bound myosin-S1 in the absence of nucleotide; the trimer contains one myosin-S1 binding site. We calculated a dissociation constant (K_d ) of 1.1 × 10^-10  m and 1.9 × 10^-10  m for binding of native F-actin and the actin-trimer to myosin-S1, respectively. EM of the actin-trimer:myosin-S1 complex demonstrated the presence of single particles of uniform size. Image reconstruction allowed a reasonable fit of the actin-trimer and myosin-S1 into the obtained density clearly showing binding of one myosin-S1 molecule to the two long-pitch actins of the trimer, supporting the kinetic data.

High flexibility of the actomyosin crossbridge resides in skeletal muscle myosin subfragment-2 as demonstrated by a new single molecule assay

Popular views of force generation in muscle indicate that a lever arm in the myosin head initiates displacement of the thin filament. However, this lever arm is attached to the thick filament backbone by a flexible combination of coiled coils and hinges in the myosin subfragment-2 (S2); therefore, efficient force generation depends on tension development in this linking structure. Herein, a single molecule assay is developed to examine the flexibility of the intact S2 relative to that of the myosin head. Fluorescently labeled myosin rod is polymerized onto a single myosin molecule that is bound to actin, and the resulting Brownian motion of the rod is analyzed at video rates by digital image processing. Complete rotations of the rod suggest significant amounts of random coil in the linking structure. The close similarity of twist rates for double-headed and single-headed myosin indicates that most of the flexibility originates at or beyond the first pitch of coiled coil in S2 and most likely at the hinge connecting S2 and the light meromyosin. The myosin head has a smaller but still detectable impact on this flexibility, since the addition of ADP to the rigor crossbridge produces differential effects on the torsional characteristics of double-headed versus single-headed myosin.