The three faces of profilin

Abstract

Actin is the major cytoskeletal protein of most eukaryotic cells, and filamentous actin structures are often the primary determinants of cell shape and movement. The polymerization and depolymerization of actin filaments inside nonmuscle cells are highly regulated, both spatially and temporally, to give the cell the ability to rearrange its cytoskeleton drastically within minutes in response to external stimuli or at certain points in the cell cycle. To harness and steerthe inherent dynamic propertiesof the actin polymer, the cell relies on a plethora of actin-binding proteins, of which over 100 have been identified. One of the most dramatic effects of actin-binding proteins in the cell is to inhibit filament formation. The critical concentration of pure ATP-bound actin is about 0.2 PM, and actin in a test tube will polymerize until the free actin monomer concentration falls to this level. Nevertheless, most cells maintain a pool of unpolymerized actin of about 50-200 PM. There are at least two plausible mechanisms for how the cell might limit filament growth: filament nucleation sites might be blocked so that the monomers have nothing to grow off, or actin monomer might be sequestered in a nonpolymerizable form so that filaments have nothing to grow with. Although there is evidence that many filament ends are blocked inside cells, this alone cannot quantitatively account for the high steady-state concentration of monomer. Furthermore, elegant microinjection experiments have shown that free nucleation sites are not elongated in resting cells (Sanders and Wang, 1990). Thus, sequestration of actin monomer is very important to the regulation of the actin cytoskeleton in vivo. Since cells often grow new filaments with astonishing rapidity, for example, during locomotion or upon platelet activation, they must also have the ability to desequester monomers efficiently in a regulated fashion. Profilin is a ubiquitous actin-monomer binding protein whose three-dimensional structure has recently been determined (Schutt et al., 1993; Vinson et al., 1993). Profilin homologs are present in organisms ranging from fungi and amoebae through trees and mammals. Evidence, discussed in this review, suggests that profilin is unique in having both positive and negative effects on polymerization and that it appears to act both as a sequestering agent and as a desequestering agent. Profilin was originally identified as a component of cell extracts that inhibited actin filament growth in vitro (Carlsson et al., 1977). For many years, profilin was assumed to be the major sequestering factor in most cells, and sequestering was considered to be profilin’s primary function (Figure 1A). However, there is not nearly enough profilin M inireview