Microfilaments (gF, ) are linear filaments built up from the globular protein actin (G-actin, molecular weight of 43 kDa). These actin molecules polymerize to form a long fiber. Two such fibers are twisted around each other in a helix to form a single actin filament with a diameter of about 7 nm. Within a cell, there are two types of actin filament organization. They form either a mesh composed of short filaments or long, parallel bundles of fibers (Fig. 2.16).
Microfilaments, together with other types of cytoskeletal filaments, are responsible for mechanical strength of the cell. They participate in such processes as cell division into two daughter cells, where their role is to anchor centrosomes at opposite poles of the cell during mitosis or in cell migration. The actin filaments participate also in the linkage of transmembrane (e.g., cell surface receptors) to cytoplasmic proteins.
Figure 2.16 The organization of actin filaments in two types of epithelial cells, namely, fibroblasts and keratinocytes, visualized using phalloidin fluorescently labeled using Alexa Fluor 488. Unpublished data courtesy of Barbara Orzechowska, IFJ PAN.