Chapter 17: Cell Organization & Movement I: Microfilaments

Cell Organization & Movement I: Microfilaments begins by examining the fundamental building block, actin, a highly conserved protein that reversibly polymerizes from globular G-actin monomers into polarized helical F-actin filaments, utilizing ATP hydrolysis to drive assembly kinetics,. The text details the critical concept of filament polarity, where the barbed (plus) end favors rapid elongation compared to the pointed (minus) end, leading to the steady-state phenomenon known as treadmilling,. A suite of actin-binding proteins is discussed for their regulatory roles: profilin and cofilin accelerate turnover, thymosin-beta4 acts as a monomer buffer, and capping proteins like CapZ and tropomodulin stabilize filament ends,. The chapter distinguishes between nucleation mechanisms, contrasting the Formin family, which facilitates the growth of long unbranched filaments via FH2 domains, with the Arp2/3 complex, which generates branched actin networks essential for structures like the leading edge of migrating cells,. Pathogenic exploitation of these mechanisms is illustrated through Listeria monocytogenes, which hijacks the host's actin machinery for intracellular propulsion. Structural diversity is further explained through cross-linking proteins such as fimbrin, alpha-actinin, and spectrin, which organize filaments into bundles or networks, and adapter proteins like the ERM family that link the cytoskeleton to the plasma membrane. The focus then shifts to myosins, a superfamily of motor proteins that convert chemical energy from ATP hydrolysis into mechanical work via a conserved power stroke mechanism,. Specific classes are detailed, including Myosin II, responsible for contractile forces in muscle and cytokinesis; Myosin V, a processive motor for organelle transport; and Myosin I, involved in membrane association,. The chapter provides an in-depth look at skeletal muscle contraction, describing the sarcomere structure where myosin thick filaments slide past actin thin filaments, regulated by calcium ion influx binding to troponin and shifting tropomyosin,. This is contrasted with smooth muscle and non-muscle contraction, which rely on thick-filament regulation via the phosphorylation of myosin light chains by MLC kinase. Finally, the complex process of cell migration is broken down into coordinated steps: membrane extension (lamellipodia/filopodia), adhesion via integrins, cell-body translocation, and rear de-adhesion. This motility is orchestrated by cell signaling pathways involving small Rho GTPases—Cdc42 (polarity), Rac (lamellipodia), and Rho (contractile stress fibers)—and can be directed by extracellular chemical cues through chemotaxis.