Actin is among the most thoroughly studied of proteins. It was first identified half a century ago as the major component of thin filaments in muscle. Work in the 1960s and 1970s showed that actin is also present in nonmuscle cells as well as in plants and protozoa. The actin-based cytoskeleton appears to be ubiquitous among eukaryotes, and indeed the invention of the actin cytoskeleton may have been a key step in the earliest history of the eukaryotic lineage. A densely written summary of the most important known properties of actin fills a good-sized volume (23). But there are major discrepancies between the well-characterized in vitro behavior of purified actin and the apparent behavior of actin filaments inside of intact, living cells.
Two discrepancies have been particularly puzzling. First, the concentration of unpolymerized actin present in most vertebrate nonmuscle cells is ∼100-fold higher than the concentration of...
