Profilin forms tetramers that bind to G-actin

Profilin binds to G-actin and affects polymerization. However, regulation of profilin function is generally unknown and controversy exists regarding profilin effects on actin polymerization. Because protein-protein interactions are implicated in many cellular responses, human platelet profilin self-association and actin inter-action was examined. Silver stained SDS-PAGE of poly-l-proline/sepharose 4B column purified profilin revealed the presence of profilin (14.8 kD) and extraneous higher bands (primarily 30 kD and 58.5 kD). Re-electrophoretic analysis of gel electroelution purified profilin yielded predominantly 14.8 kD and 58.5 kD proteins. Rabbit IgG antibodies made against gel electroelution-purified profilin recognized all profilin sizes on immunoblots. Capillary electrophoresis of profilin in solution produced a single peak that resolved into three distinct peaks upon addition of reducing agent or high salt conditions. Further, G-actin did not bind to 14.8 kD profilin on immunoblot overlay assays, but surprisingly bound only to 58.5 kD profilin. The data indicate that monomeric profilin forms tetramers which are the relevant high affinity actin-binding form.