The formation and growth of actin filaments at the leading edge of a cell are assisted by various actin-binding proteins. The actin-related proteins (ARPs) promote the formation of a web of branched actin filaments in lamellipodia.

ARPs form complexes that bind to the sides of existing actin filaments and nucleate the formation of new filaments, which grow out at an angle to produce side branches. With the aid of additional actin-binding proteins, this web undergoes continual assembly at the leading edge and disassembly further back, pushing the lamellipodium forward.

The other kind of cell protrusion, the filopodium, depends on formin, a nucleating protein that attaches to the growing plus ends of actin filaments and promotes the addition of new monomers to form straight, unbranched filaments.

Formins are also used elsewhere to assemble unbranched filaments, such as in the contractile ring that pinches a dividing animal cell in two.