(5The nucleation of actin filaments at the pm is frequently regulated by external signals, allowing the cell to change its shape and stiffness rapidly in response to changes in its external environment This nucleation is catalyzed by a complex of proteins that includes two actin-related proteins, or ARPS(Arp2 and arp3).(5)The nucleation of actin filaments at the PM is frequently regulated by external signals, allowing the cell to change its shape and stiffness rapidly in response to changes in its external environment. This nucleation is catalyzed by a complex of proteins that includes two actin-related proteins, or ARPs(Arp2 and Arp3)