Frantz et al. demonstrate how events coincide at the cell's leading edge to regulate the activity of actin-severing cofilin.
Cofilin promotes the formation of actin filaments at the front of motile cells by binding to existing filaments and cutting them, creating barbed ends for new filament nucleation. Cofilin activation requires Ser3 dephosphorylation; however, this alone is not sufficient. Additional mechanisms—including deprotonation of His133 (by increasing cellular pH), and release from membrane phospholipid, PI(4,5)P2—might also be necessary. Cofilin presumably acts as a “coincidence detector” for these signals, explains author Diane Barber.
The question was, how do these individual regulatory events combine. Increased pH by H+ efflux had been suggested for the His133 deprotonation, and the authors now confirm that, in response to migratory cues, H+ efflux by the mammalian Na-H exchanger (NHE1) was required for increasing actin barbed end formation by cofilin. As for PI(4,5)P2 release, the authors found that His133 deprotonation in fact decreased cofilin binding to the phospholipid. Thus, these two regulatory steps are related.
The authors went on to determine how events combine at the structural level. Using computational modeling, NMR, and site-directed mutagenesis, they showed that dephosphorylation of Ser3 unblocked an amino-terminal actin-binding site on cofilin. Whereas at the other end of the protein, the decreased binding of PI(4,5)P2, after His133 deprotonation, freed up a second actin-binding site.
pH-dependent binding of membrane phospholipids is a growing phenomenon in protein biology, suggested to be a negative regulatory mechanism in some instances—as now shown for cofilin. It may be particularly relevant at the cell's leading edge where actin regulators can be kept close at hand, but remain inactive until needed.