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Microfilament functions include cytokinesis, amoeboid movement, cell motility, changes in cell shape, endocytosis and exocytosis, cell contractility, and mechanical stability. Microfilaments are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces.
Microfilament Polymerization. Microfilament polymerization is divided into three steps. The nucleation step is the first step, and it is the rate limiting and slowest step of the process. Elongation is the next step in this process, and it is the rapid addition of actin monomers at both the plus and minus end of the microfilament.
Microfilament formation showing the polymerization mechanism for converting G-actin to F-actin; note the hydrolysis of the ATP. Actin filaments are often rapidly assembled and disassembled, allowing them to generate force and support cell movement. [112] Assembly classically occurs in three steps.
Cell surface (cortical) actin remodeling is a cyclic (9-step) process where each step is directly responsive to a cell signaling mechanism. Over the course of the cycle, actin begins as a monomer, elongates into a polymer with the help of attached actin-binding-proteins, and disassembles back into a monomer so the remodeling cycle may commence again.
The first step in actin polymerization, after polymerization is initiated, is the deprotonation of the thiol group of G-actin. This renders the sulfur atom charged and makes it available for actin polymerization. If cytochalasin B is present in the cell, the deprotonation of thiol is competed.
The γ-tubulin combines with several other associated proteins to form a lock washer-like structure known as the "γ-tubulin ring complex" (γ-TuRC). This complex acts as a template for α/β-tubulin dimers to begin polymerization; it acts as a cap of the (−) end while microtubule growth continues away from the MTOC in the (+) direction. [28]
The cytoskeleton is a highly dynamic part of a cell and cytoskeletal filaments constantly grow and shrink through addition and removal of subunits. Directed crawling motion of cells such as macrophages relies on directed growth of actin filaments at the cell front (leading edge).
CapZ plays a role in cell movement (cell crawling) by controlling the lengths of the microfilaments. When CapZ is inhibited by regulating factors, microfilament polymerization or depolymerization occurs allowing lamellipodia and filopodia to grow out or retract. This polymerization and depolymerization gives the cell the appearance of crawling.