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• Cell Polarity and • the Actin • Cytoskeleton Chapter 16 in Alberts Cell Polarity Defines Cell Shape and Behavior How do cells assume a particular shape? How do cells change their shape? Cell Polarity is Connected to the Cell Cycle G1 Polarity Establishment Mitotic Exit S M Rhodamine Falloidin G2 Polarized Secretion Rhodamine falloidin staining of yeast cells to show actin Actin patches and Cables Figure 16-18. Molecular Biology of the Cell, 4th Edition The Cytoskeleton is a cytoplasmic structure Of polymerized protein filaments Actin Actin filaments = microfilaments 2 stranded helical polymers, they’re flexible Actin subunits polymerize into filaments Actin Polymerization is stimulated once nucleation occurs Salt induces Polymerization In vitro Actin is and ATP binding protein Actin binds to Other Proteins That Influence its Function CHI- complex haploinsufficiency and actin haploinsufficiency has been gaining in appreciation as an important influence on human disease human individuals may have as many as 45 CHI gene pairs Actin Filament Assembly, Growth, Branching, Capping and Disassembly are Controlled by Actin Binding Proteins The ARP (Actin-Related Proteins) complex Regulates Actin Polymerization Arp2 and Arp3 cannot form polymers due to differences in their minus ends Filament Elongation is Modified By Proteins that Bind to Free Subunits The ARP complex Binds nucleates filaments more efficiently When bound to the side of a preexisting Actin Filament Capping Proteins Prevent Polymerization Actin polymerization is induced by Arp 2/3 protein complex at the surface of Listeria monocytogenes Matthew D. Welch et al. The pathogenic bacterium Listeria monocytogenes is capable of directed movement within the cytoplasm of infected host cells. Propulsion is thought to be driven by actin polymerization at the bacterial cell surface and moving bacteria leave in their wake a tail of actin filaments. Actin assembly by L. monocytogenes requires the bacterial surface protein ActA and protein components present in host cell cytoplasm. We have purified an eight-polypeptide complex that possesses the properties of the host-cell actin polymerization factor. The pure complex is sufficient to initiate ActAdependent actin polymerization at the surface of L. monocytogenes, and is required to mediate actin tail formation and motility. Two subunits of this protein complex are actin-related proteins (ARPs) belonging to the Arp2 and Arp3 subfamilies. The Arp3 subunit localizes to the surface of stationary bacteria and the tails of motile bacteria in tissue culture cells infected with L. monocytogenes; this is consistent with a role for the complex in promoting actin assembly in vivo. The activity and subunit composition of the Arp2/3 complex suggests that it forms a template that nucleates actin polymerization.