Although actin is found throughout the cytoplasm of a eukaryotic cell, in many cells it is highly concentrated in a layer just beneath the plasma membrane. In this region, called the cell cortex, actin filaments are linked by actin-binding proteins into a meshwork that supports the plasma membrane and gives it mechanical strength.
B.
In human red blood cells, for example, a simple and regular network of fibrous proteins—including actin and spectrin filaments—attaches to the plasma membrane, providing the support necessary for the cells to maintain their simple discoid shape.
C.
In other animal cells, the cortex includes a much denser network of actin filaments that are cross-linked into a three-dimensional meshwork.
D.
The rearrangements of actin filaments within the cortex provide much of the molecular basis for changes in both cell shape and cell locomotion.