Motors of the myosin family can also participate in cell locomotion, by propelling actin filaments past cellular structures and by causing localized contractions. The movement of a cell across the substratum involves the extension of a lamellapodium, transient attachment to the substratum, contraction of the rear of the cell and finally release of rearward contacts. The Arp2/3 complex is a critical regulator of actin dynamics that forms branched networks necessary to make lamellapodia protrude. The Arp2/3 complex nucleates actin assembly in a highly regulated manner that is influenced by physiological stimuli. Arp2/3 can be ‘off’, and then turned on by N-WASp and Cdc42 (a small GTP-binding protein related to Ras), which allows Arp2/3 to nucleate actin assembly. Formins are also activated by small GTP-binding proteins. Focal activation of actin assembly right next to the plasma membrane leads to formation of filopodia and other structures that containing parallel arrays of F-actin. Actin polymerization-derived forces underlie important aspects of pathogenesis of human disease agents such as the bacteria Listeria monocytogenes , Shigella and Salmonella . These bacteria contain proteins that are molecular mimics of host cell regulatory proteins that control actin dynamics under normal conditions. Shigella and Salmonella force cells that are not normally phagocytic to engulf them, whereas Listeria uses actin polymerization to power its movement inside cells.
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