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Background:  Cofilin is a low-molecular weight actin-modulating protein, and is structurally and functionally conserved in eucaryotes from yeast to mammals. The functions of cofilin appear to be regulated by phosphorylation and dephosphorylation.

Results:  A proteolytic study of phosphorylated porcine cofilin and expression of a mutated cofilin in cultured cells revealed that Ser-3 is the unique phosphorylation site. Phosphorylated cofilin was found not to bind to either F-or G-actin while unphosphorylated cofilin binds to both. S3D-cofilin, in which Ser-3 was replaced with Asp, did not bind in vitro to actin while S3A-cofilin did. The transient overexpression of wild-type or S3A-cofilin in cultured cells caused disruption of pre-existing actin structures and induced cytoplasmic actin bundles. Heat shock-induced nuclear or NaCl buffer-induced cytoplasmic actin/cofilin rods contained the expressed cofilin. In contrast, the overexpression of S3D-cofilin did not alter the actin structures. Induced actin rods did not contain S3D-cofilin. S3D-porcine cofilin did not complement the lethality associated with Δcof1 mutations in Saccharomyces cerevisiae while wild-type and S3A-cofilin did. Furthermore, we found that S2A/S4D- and S2D/S4D-yeast cofilin mutants were not viable.

Conclusion:  We conclude that the function of cofilin is negatively regulated in vivo by phosphorylation of Ser-3 and that cells require the functions of unphosphorylated cofilin for viability.