Introduction to Actin

In the cytoskeleton, actins are the building blocks of cytoskeletal microfilaments. Actin monomers have a round shape and are called globular or G-actin.

These monomers polymerize head-to-tail forming a tight, right-handed helical structure called filamentous or F-actin.

Each actin subunit has an outer and an inner domain joined by a smaller linker helix. Such an arrangement forms two clefts: the upper cleft binds an ATP and a magnesium ion. The lower hydrophobic cleft is specific for actin-binding proteins.

G-actin has a low ATPase activity that is enhanced in F-actin. When an ATP-G-Actin complex binds F-actin, ATP is hydrolyzed to ADP and phosphate, forming a highly stable filament.

The ATP-bound growing end of F-actin is called the plus-end, while the other end, bound to ADP,  is the minus-end.

Actins have different isoforms, broadly classified into alpha, beta, and gamma. They are expressed in different cell types, such as α-actin in contractile muscle fibers, β-actin in cell cortex, and γ- actin in smooth muscle fibers.