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The Cytoskeleton
The cytoskeleton of the cell plays many important roles. It provides structure and support for the cell allowing cells to take on a variety of shapes and to resist the forces that act on the cell. It also provides an internal newtork in the cell that allows proteins and organelles to be transported to specific locations within the cell. The cytoskeleton also provides the driving force behind cell movement.
The cytoskeleton is composed mainly of three types of filament - actin filaments, intermediate filaments and microtubules. Each of these filaments is essentially a polymer composed of numerous subunits. This allows rapid assembly and disassembly of the polymer filaments - by adding or subtracting subunits - according the changing requirements of the cell. An example of the actin and microtubule network in a dividing cell is shown below.
Cell migration is driven by extensive polymerisation of actin filaments at the leading edge of the cell (see movie).
Actin monomers are added to the front end of the actin filament and taken off the back end of the filament (a process known as treadmilling). In this manner the actin filament grows in the direction of cell movement, pushing the plasma membrane forward and often leading to the formation of a structure called the lamellipodium. The lamellipodium is a attached to the substrate through the formation f adhesion complexes which in turn allow the cell to generate traction and to pull itself forward. The continuous polymerisation of actin filaments is required to produce cell movement and these actin filaments must be linked to the plasma membrane or to adhesion complexes. The regulation of actin dynamics is complex and tightly controlled and involves a diverse range of proteins including the Arp2/3 complex (which controls branching of actin filaments), cofilin (which depolymerises actin filaments) and Rho GTPases such as Rac (regulates lamellipodia formation), RhoA (regulates actin strss fibre formation) and cdc42 (regulates filopodia formation).