No abstract

The large GTPase dynamin, best known for its activities that remodel membranes during endocytosis, also regulates F-actinrich structures, including podosomes, phagocytic cups, actin comet tails, subcortical ruffles, and stress fibers. The mechanisms by which dynamin regulates actin filaments are not known, but an emerging view is that dynamin influences F-actin via its interactions with proteins that interact directly or indirectly with actin filaments. We show here that dynamin2 GTPase activity remodels actin filaments in vitro via a mechanism that depends on the binding partner and F-actin-binding protein, cortactin. Tightly associated actin filaments crosslinked by dynamin2 and cortactin became loosely associated after GTP addition when viewed by transmission electron microscopy. Actin filaments were dynamically unraveled and fragmented after GTP addition when viewed in real time using total internal reflection fluorescence microscopy. Cortactin stimulated the intrinsic GTPase activity of dynamin2 and maintained a stable link between actin filaments and dynamin2, even in the presence of GTP. Filaments remodeled by dynamin2 GTPase in vitro exhibit enhanced sensitivity to severing by the actin depolymerizing factor, cofilin, suggesting that GTPase-dependent remodeling influences the interactions of actin regulatory proteins and F-actin. The global organization of the actomyosin cytoskeleton was perturbed in U2-OS cells depleted of dynamin2, implicating dynamin2 in remodeling actin filaments that comprise supramolecular F-actin arrays in vivo. We conclude that dynamin2 GTPase remodels actin filaments and plays a role in orchestrating the global actomyosin cytoskeleton.

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Studies of the surface damage caused by the sliding of clean metals on one another show that penetration and distortion occur to some depth beneath the surface. Micro-examination shows that welding of the metals takes place quite readily even at low speeds of sliding when the surface temperature-rise due to frictional heat cannot be very high. In some cases the welded junctions may pluck out portions of the harder metal. These and other results have led to a more quantitative theory of metallic friction. It is suggested that in general the frictional force between clean metal surfaces is made up of two parts. The first is the force required to shear the metallic junctions formed between the surfaces; the second is the ploughing force required to displace the softer metal from the path of the harder. By using steel sliders of various shapes and sizes on a soft metal like indium, these two factors have been estimated separately, and it is shown that an approximate calculation of the friction between metal surfaces may be made in terms of the known physical properties of the metals. The effect of surface contamination is also discussed.

No abstract