Separation of propulsive and adhesive traction stresses in locomoting keratocytes

Strong, actomyosin-dependent, pinching tractions in steadily locomoting (gliding) fish keratocytes revealed by traction imaging present a paradox, since only forces perpendicular to the direction of locomotion are apparent, leaving the actual propulsive forces unresolved. When keratocytes become transiently "stuck" by their trailing edge and adopt a fibroblast-like morphology, the tractions opposing locomotion are concentrated into the tail, leaving the active pinching and propulsive tractions clearly visible under the cell body. Stuck keratocytes can develop approximately 1 mdyn (10,000 pN) total propulsive thrust, originating in the wings of the cell. The leading lamella develops no detectable propulsive traction, even when the cell pulls on its transient tail anchorage. The separation of propulsive and adhesive tractions in the stuck phenotype leads to a mechanically consistent hypothesis that resolves the traction paradox for gliding keratocytes: the propulsive tractions driving locomotion are normally canceled by adhesive tractions resisting locomotion, leaving only the pinching tractions as a resultant. The resolution of the traction pattern into its components specifies conditions to be met for models of cytoskeletal force production, such as the dynamic network contraction model (Svitkina, T.M., A.B. Verkhovsky, K.M. McQuade, and G.G. Borisy. 1997. J. Cell Biol. 139:397-415). The traction pattern associated with cells undergoing sharp turns differs markedly from the normal pinching traction pattern, and can be accounted for by postulating an asymmetry in contractile activity of the opposed lateral wings of the cell.

Modulatory role of drebrin on the cytoskeleton within dendritic spines in the rat cerebral cortex

Morphological changes in the dendritic spines have been postulated to participate in the expression of synaptic plasticity. The cytoskeleton is likely to play a key role in regulating spine structure. Here we examine the molecular mechanisms responsible for the changes in spine morphology, focusing on drebrin, an actin-binding protein that is known to change the properties of actin filaments. We found that adult-type drebrin is localized in the dendritic spines of rat forebrain neurons, where it binds to the cytoskeleton. To identify the cytoskeletal proteins that associated with drebrin, we isolated drebrin-containing cytoskeletons using immunoprecipitation with a drebrin antibody. Drebrin, actin, myosin, and gelsolin were co-precipitated. We next examined the effect of drebrin on actomyosin interaction. In vitro, drebrin reduced the sliding velocity of actin filaments on immobilized myosin and inhibited the actin-activated ATPase activity of myosin. These results suggest that drebrin may modulate the actomyosin interaction within spines and may play a role in the structure-based plasticity of synapses.

Myosin is a component of the cell surface of cultured cells

Antisera to myosin from mouse L929 cells, mouse uterus, and rabbit skeletal muscle have been prepared in goats. Each antiserum shows specific reaction by immunodiffusion only to the myosin against which it was prepared. Antiserum against L cell myosin agglutinates L929 cells and shows surface localization in unfixed cells by indirect immunofluorescence. This reaction is prevented by immunoabsorption with L cell myosin, but not by absorption with mouse uterine or rabbit skeletal muscle myosins. Elution of the antibody from L cell myosin antiserum that absorbs to fixed L cells yields antibody reactive to L cell myosin in immunodiffusion. At the ultrastructural level, surface localization is also evident with a peroxidase bridge immunocytologic method as a uniform distribution of surface label. Antiserum against L cell myosin also shows localization on the surface of other cells of rodent origin (3T3-4, NRK, MNRK, KNRK, and MEF) and HeLa cells, but not with some other cells of human origin (ALL and WI-38). The reactivity with antibody of the plasma membrane myosin is unaffected by trypsin treatment or fixation with 0.5-2.0% formalin. These results strongly suggest that myosin in cultured nonmuscle cells is a component of the plasma membrane, a finding of possible significance in studying the mechanisms regulating the motility, morphology, and growth of these cells.

Filament formation by purified Physarum myosin

Physarum myosin can be separated from actomyosin by ultracentrifugation, and purified by gel filtration. Unlike actomyosin, myosin is soluble in 0.05 M KCl in the pH range of 6-7. However, in the absence of actin, the slime mold myosin can be precipitated in 0.05 M KCl by the addition of millimolar concentrations of CaCl(2). The precipitates consist of aggregated, short bipolar filaments. Magnesium has a similar effect, but results in the precipitation of more loosely packed aggregates. The length of the compact filaments is 0.45 mum; thus, predominantly tail-to-tail, but also some head-to-tail, interactions occur under these conditions. Since the size and shape of these thick filaments are close to those seen in fixed and sectioned ameboid cells and in platelets, all of these filaments are probably composed of myosins.