Intracellular pH modulation of ADF/cofilin proteins

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Abstract
The ADF/cofilin (AC) proteins are necessary for the high rates of actin filament turnover seen in vivo. Their regulation is complex enough to underlie the precision in filament dynamics needed by stimulated cells. Disassembly of actin by AC proteins is inhibited in vitro by phosphorylation of ser3 and pHi) is reduced from 7.4 to 6.6; (3) Covariance and colocalization analyses of immunostained endogenous proteins show that ADF partitions more with monomeric actin and less with polymeric actin when pHi increases. However, the distribution of cofilin, a less pH‐sensitive AC in vitro, does not change with pH; (4) Only the unphosphorylatable AC mutant (A3), when overexpressed as a GFP chimera, uniquely produces aberrant cellular phenotypes and only if the pH is shifted from 7.1 to 6.6 or 7.4. A mechanism is proposed that explains why AC(A3)‐GFP and AC(wt)‐GFP chimeras generate different phenotypes in response to pH changes. Phospho‐AC levels increase with cell density, and in motile cells, phospho‐AC increases with alkalization, suggesting a homeostatic mechanism that compensates for increased AC activity and filament turnover. These results show that the behavior of AC proteins with pH‐sensitivity in vitro is affected by pH in vivo. Cell Motil. Cytoskeleton 47:319–336, 2000.