The actin-severing and -depolymerizing activities of cofilin help to keep the actin cytoskeleton dynamic, which is crucial for cellular processes such as cell division and motility. Phosphorylation of cofilin on Ser3 by LIM kinase (LIMK) inhibits its activity, but a broadly expressed, counteractive phosphatase has remained elusive. Gary Bokoch's group, though, has unearthed chronophin (CIN), a haloacid dehydrogenase (HAD)-type phosphatase that dephosphorylates cofilin.

CIN was isolated from bovine brain cytosol on the basis of its ability to dephosphorylate Ser3-phosphocofilin, and sequence homology showed the presence of orthologues from bacteria to humans. CIN protein is widely expressed throughout human tissues; at the subcellular level, it colocalizes with F-actin and cofilin in membrane ruffles and lamellipodia, and mirrors the dynamic distribution of cofilin to regions of actin dynamics during mitosis.

Having confirmed that CIN had intrinsic phosphatase activity, which was abolished by the mutation of a crucial aspartate in the N-terminal HAD motif DXDXT, the authors then verified that recombinant CIN could dephosphorylate cofilin. CIN showed no activity towards tyrosine-phosphorylated peptide substrates. Next, Bokoch's group showed that transfected wild-type CIN decreased the steady-state levels of phosphocofilin in HeLa cells by ∼50%, and markedly induced cofilin dephosphorylation during mitosis. Conversely, when CIN was depleted from HeLa cells using RNA inhibition (RNAi), the levels of phosphorylated cofilin increased.

But how does CIN actually affect cells? Expressing recombinant CIN for prolonged periods of time caused cells to lose their cortical actin cytoskeletons, decrease their amount of stress fibres, and eventually round up and detach from the substrate. Over a shorter time period, membrane ruffling increased. A phosphatase-dead CIN construct increased actin polymerization, which is similar to the effect of increasing LIMK activity.

As interfering with CIN levels also perturbed cell division (causing multinucleate cells to form), the authors studied the cofilin phosphorylation profile in response to CIN during mitosis. As noted previously, wild-type CIN reduced cofilin phosphorylation during mitosis. Phosphatase-dead CIN caused levels of phosphorylated cofilin to accumulate, which resulted in prolonged mitotic progression and impaired cytokinesis — in particular, contractile-ring defects. Such phenotypes are similar to those that arise from defective cofilin function in lower organisms.

So CIN is required to control cofilin activity during cell division. On the basis of the authors' other observations and the expression patterns of CIN, this unusual HAD-type phosphatase probably influences motility, polarity and membrane dynamics as well.