It occurs as a monomer, G-actin, a globular protein, and in living cells as a polymer, F-actin, which resembles two strings of beads twisted around each other into thin filaments.

Severing proteins induce kinks in the filaments and led to the formation of short filaments that rapidly depolymerize or give rise to new filaments.

The depolymerization of proteins may lead to fragments of filaments which form smaller filaments that depolymerize quickly or polymerize into new filaments.