Summary Lamellipodia, the sheet-like protrusions of motile cells, consist of networks of actin filaments (F-actin) regulated by the ordered assembly from and disassembly into actin monomers (G-actin). Traditionally, G-actin is thought to exist as a homogeneous pool. Here, we show that there are two functionally and molecularly distinct sources of G-actin that supply lamellipodial actin networks. G-actin originating from the cytosolic pool requires the monomer binding protein thymosin β4 (Tβ4) for optimal leading edge localization, is targeted to formins, and is responsible for creating an elevated G/F-actin ratio that promotes membrane protrusion. The second source of G-actin comes from recycled lamellipodia F-actin. Recycling occurs independently of Tβ4 and appears to regulate lamellipodia homeostasis. Tβ4-bound G-actin specifically localizes to the leading edge because it doesn’t interact with Arp2/3-mediated polymerization sites found throughout the lamellipodia. These findings demonstrate that actin networks can be constructed from multiple sources of monomers with discrete spatiotemporal functions.

B. stearothermophilus tryptophanyl-tRNA synthetase catalysis proceeds via high-energy protein conformations. Unliganded MD trajectories of the pretransition-state complex with Mg(2+)ATP and the (post) transition-state analog complex with adenosine tetraphosphate relax rapidly in opposite directions, the former regressing, the latter progressing along the structural reaction coordinate. The two crystal structures (rmsd 0.7 A) therefore lie on opposite sides of a conformational free-energy maximum as the chemical transition state forms. SNAPP analysis illustrates the complexity of the associated long-range conformational coupling. Switching interactions in four nonpolar core regions are locally isoenergetic throughout the transition. Different configurations, however, propagate their effects to unfavorable, longer-range interactions at the molecular surface. Designed mutation shows that switching interactions enhance the rate, perhaps by destabilizing the ground state immediately before the transition state and limiting nonproductive diffusion before and after the chemical transition state, thereby reducing the activation entropy. This paradigm may apply broadly to energy-transducing enzymes.

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A cyclic process of membrane-cortex compression and dilation generates a traveling wave of cortical actin density that in turn generates oscillations in cell morphology.

The Picornaviridae are a diverse family of positive-strand RNA viruses that includes numerous human and veterinary pathogens 1 . Among these, hepatitis A virus (HAV), a common cause of acute hepatitis in humans, is unique in that it is hepatotropic and released from hepatocytes without lysis in small vesicles resembling exosomes 2 , 3 . These quasi-enveloped virions (eHAV) are infectious and the only form of virus detected in blood during acute infection 2 . By contrast, non-enveloped, naked virions (nHAV) are shed in feces, stripped of membranes by bile salts during passage through bile ducts to the gut 4 . How these two distinct types of infectious hepatoviruses enter cells to initiate infection is enigmatic. Here we describe a genome-wide forward screen that identified glucosylceramide synthase (UGCG) and other components of the ganglioside synthetic pathway as crucial host factors required for cellular entry by hepatoviruses. We show that gangliosides, preferentially disialogangliosides, function as essential endolysosome receptors required for infection by both naked and quasi-enveloped virions. In the absence of gangliosides, both virion types are efficiently internalized through endocytosis, but capsids fail to uncoat and accumulate within LAMP1 + endolysosomes. Gangliosides relieve this block, binding the capsid at low pH and facilitating a late step in entry involving uncoating and delivery of the RNA genome to the cytoplasm. These results reveal an atypical cellular entry pathway for hepatoviruses that is unique among picornaviruses.