Chapter 14: The Cytoskeleton & Cell Movement

Regulation of actin dynamics is mediated by various binding proteins, such as formins and the Arp2/3 complex for nucleation, profilin for monomer exchange, and cofilin for filament severing. The text details how actin filaments organize into bundles and networks to support the plasma membrane, forming the cell cortex (exemplified by the spectrin-actin network in erythrocytes) and specialized surface protrusions like microvilli. It further explores cell-matrix and cell-cell adhesions through focal adhesions and adherens junctions, involving integrins, cadherins, and catenins. The chapter then transitions to myosin motors, explaining the sliding filament model of muscle contraction driven by calcium-regulated interactions between myosin II and actin within sarcomeres, as well as nonmuscle contractile assemblies like stress fibers and the contractile ring during cytokinesis. Unconventional myosins are highlighted for their role in vesicle transport. The second major cytoskeletal component, microtubules, is described as rigid hollow rods composed of tubulin dimers that exhibit dynamic instability driven by GTP hydrolysis. This section covers microtubule nucleation at centrosomes via the gamma-tubulin ring complex, regulation by microtubule-associated proteins (MAPs) like tau, and the establishment of cell polarity in neurons. Molecular motors associated with microtubules, specifically kinesins and dyneins, are explained regarding their directional transport of organelles and their critical function in the beating of cilia and flagella (characterized by the 9 plus 2 axoneme structure). The chapter outlines the dramatic reorganization of microtubules during mitosis to form the mitotic spindle, separating chromosomes through anaphase A and B movements. Finally, the text examines intermediate filaments, including keratins, vimentin, and neurofilaments, which lack polarity and motor proteins but provide essential mechanical strength to cells and tissues by anchoring to desmosomes and hemidesmosomes.