HOW PROFILIN PROMOTES ACTIN FILAMENT ASSEMBLY IN THE PRESENCE OF THYMOSIN-BETA-4

The role of profilin in the regulation of actin assembly has been reex amined. The affinity of profilin for ATP-actin appears 10-fold higher than previously thought. in the presence of ATP, the participation of the profilin-actin complex to filament elongation at the barbed end is linked to a decrease in the steady-state concentration of globular ac tin. This surprising effect is made possible by the involvement of the irreversible ATP hydrolysis accompanying actin polymerization. As a c onsequence, in the presence of thymosin beta4 (Tbeta4), low amounts of profilin promote extensive actin assembly off of the pool of actin-Tb eta4 complex. When barbed ends are capped, profilin simply sequesters globular actin. A model is proposed for the function of profilin in ac tin-based motility.