Profilin competes with Thymosin for binding to actin monomers and promotes assembly

In cells G-actin is 1000x more concentrated than the critical concentration for actin filament formation. However most G-actin is bound to thymosin-b4 and this cannot be incorporated into filaments. Raising thymosin-b4 levels by micro-injection inhibits actin filament assembly by sequestering actin from dissociation filaments. Profilin binds G-actin more weakly than thymosin-b4, but stronger than actin + ends. Uniquely it allows ADP/ATP exchange and can promote ATP replacement and promotes actin incorporation into filaments. Profilin is mostly at the plasma membrane bound to PIP2