Profilin-actin complexes directly elongate actin filaments at the barbed end.

We demonstrate that the profilin-G-actin complex can elongate actin filaments directly at the barbed end but cannot bind to the pointed end. During elongation, the profilin-actin complex binds to the barbed filament end, whereupon profilin is released, leaving the actin molecule behind. This was first proposed by Tilney Tilney, L. G., et al. (1983) J. Cell Biol. 97, 112-124] and demonstrated by Pollard and Cooper (1984) Biochemistry 23, 6631-6641] by electron microscopy. We show that a model without any outside energy supply, in contrast to the mechanism proposed by Pollard and Cooper, can be fitted to our and their Kaiser et al. (1986) J. Cell Biol. 102, 221-226] findings. Input of outside energy is necessary only if profilin-mediated elongation continues after free G-actin has been lowered to or below the critical concentration observed at the barbed end in the absence of profilin.