Microfilaments

Microfilaments are variable length cytoplasmic strands made by the polymerization of the protein actin. Monomeric actin, G-actin, is very abundant within the cytoplasm and can polymerize to its filamentous form, F-actin. To do this, it requires co-factors such as ATP and magnesium ions.

Within the cell, microfilaments have lots of functions:

• provide cell strength; a network of microfilaments is present throughout the cytoplasm bound together by cross-linking proteins; this network forms the rigid cell cortex beneath the membrane. The network is dynamic and can be disassembled by the action of calcium-binding enzymes.
• can provide movement e.g. extension of cell processes, movement of microvilli, transit of membrane proteins or streaming of entire cell:
  • may occur through free myosin, calcium and tropomysoin within the cytoplasm interacting with actin, as in muscle
  • or via enzymes which interact with actin to produce movement by the concomitant breakdown of ATP
• provide adherence for body of cell to environment:
  • a component of (hemi-)desmosomes
  • it binds indirectly via proteins vinculin and alpha-actinin to transmembrane proteins which are further linked to the extracellular matrix