Muscle contraction mechanism based on actin filament rotation

Chirality of the cytoskeleton in the origins of cellular asymmetry

Self-assembly of two important components of the cytoskeleton of eukaryotic cells, actin microfilaments and microtubules (MTs) results in polar filaments of one chirality. As is true for bacterial flagella, in actin microfilaments, screw direction is important for assembly processes and motility. For MTs, polar orientation within the cell is paramount. The alignment of these elements in the cell cytoplasm gives rise to emergent properties, including the potential for cell differentiation and specialization. Complex MTs with a characteristic chirality are found in basal bodies and centrioles; this chirality is preserved in cilia. In motile cilia, it is reflected in the direction of the effective stroke. The positioning of the basal body or cilia on the cell surface depends on polarity proteins. In evolution, survival depends on global polarity information relayed to the cell in part by orientation of the MT and actin filament cytoskeletons and the chirality of the basal body to determine left and right coordinates within a defined anterior-posterior cell and tissue axis.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Cortical actin dynamics driven by formins and myosin V

Cell morphogenesis requires complex and rapid reorganization of the actin cytoskeleton. The budding yeast Saccharomyces cerevisiae is an invaluable model system for studying molecular mechanisms driving actin dynamics. Actin cables in yeast are formin-generated linear actin arrays that serve as tracks for directed intracellular transport by type V myosins. Cables are constantly reorganized throughout the cell cycle but the molecular basis for such dynamics remains poorly understood. By combining total internal reflection microscopy, quantitative image analyses and genetic manipulations we identify kinetically distinct subpopulations of cables that are differentially driven by formins and myosin. Bni1 drives elongation of randomly oriented actin cables in unpolarized cells, whereas both formins Bnr1 and Bni1 mediate slower polymerization of cables in polarized cells. Type V myosin Myo2 surprisingly acts as a motor for translational cable motility along the cell cortex. During polarization, cells change from fast to slow cable dynamics through spatio-temporal regulation of Bni1, Bnr1 and Myo2. In summary, we identify molecular mechanisms for the regulation of cable dynamics and suggest that fast actin reorganization is necessary for fidelity of cell polarization.

Site-directed spectroscopic probes of actomyosin structural dynamics

Spectroscopy of myosin and actin has entered a golden age. High-resolution crystal structures of isolated actin and myosin have been used to construct detailed models for the dynamic actomyosin interactions that move muscle. Improved protein mutagenesis and expression technologies have facilitated site-directed labeling with fluorescent and spin probes. Spectroscopic instrumentation has achieved impressive advances in sensitivity and resolution. Here we highlight the contributions of site-directed spectroscopic probes to understanding the structural dynamics of myosin II and its actin complexes in solution and muscle fibers. We emphasize studies that probe directly the movements of structural elements within the myosin catalytic and light-chain domains, and changes in the dynamics of both actin and myosin due to their alternating strong and weak interactions in the ATPase cycle. A moving picture emerges in which single biochemical states produce multiple structural states, and transitions between states of order and dynamic disorder power the actomyosin engine.