Cytoplasmic localization and nuclear transport of cofilin in cultured myotubes.

We used immunofluorescence methods to examine the cellular distribution of cofilin in chicken myotubes in primary culture. Cofilin showed mainly diffuse distribution in the cytoplasm except for rather strong staining around the nuclei and faint striated patterns along myofibrils, but did not stain inside the nuclei. Neither stress fiber-like structures nor myofibrils were clearly stained. In the presence of 10% dimethyl sulfoxide (DMSO), intranuclear actin-cofilin rods, which were composed of alpha-actin isoform and cofilin, were formed in all the nuclei of individual myotubes. In the cofilin sequence, a putative nuclear localization signal (NLS) was observed. We examined the NLS activity of this portion by using a synthetic peptide corresponding to the putative NLS. When the NLS peptide conjugated with bovine serum albumin was microinjected into the cytoplasm of myotubes, it was rapidly accumulated into the nuclei. The same result was obtained with in vitro a nuclear protein import assay system with digitonin-permeabilized myotubes. Therefore, we suggest that this portion is responsible for the nuclear transport of cofilin. In myotubes, the majority of cofilin was present in an unphosphorylated form and this form remained unchanged after the DMSO treatment. Thus, we suggest that the phosphorylation of cofilin itself is not directly involved in its nuclear transport at least in myotubes.