Profilin, an Essential Control Element for Actin Polymerization

This chapter reviews some aspects of the biochemistry and cellular function of profilin, focussing on its role as a control component of actin polymerization. Signalling-dependent changes in cell behaviour are direct consequences of a force-generating remodelling of the actin microfilament system at the inner surface of the plasma membrane. Characteristic for this sub-membraneous region is the enrichment of actin filaments in highly ordered bundles and sheets of filaments. These filaments, which have their fast polymerizing (+)-ends facing the lipid bilayer, are under constant turnover, with ATP-containing actin monomers being added at their (+)-ends, and ADP-actin monomers dissociating from the (-)-ends in a treadmilling process regulated by a number of actin-binding proteins. Here, profilin comes into play as one of the key regulators of actin filament formation. The protrusive surface activity, typically seen after receptor stimulation, is caused by local incorporation of actin from profilin-actin into the ends of growing filaments of filopodia and lamellipodia. Thus, the function of profilin is primarily integrated with the force-generating microfilament apparatus at the cell periphery. In addition to actin, profilin also binds polyphosphoinositides and proline-rich ligands.