Differential localization of non-muscle myosin II isoforms and phosphorylated regulatory light chains in human MRC-5 fibroblasts.

We investigated the localization of non-muscle myosin II isoforms and mono- (at serine 19) and diphosphorylated (at serine 19 and threonine 18) regulatory light chains (RLCs) in motile and non-motile MRC-5 fibroblasts. In migrating cells, myosin IIA localized to the lamella and throughout the posterior region. Myosin IIB colocalized with myosin IIA to the posterior region except at the very end. Diphosphorylated RLCs were detected in the restricted region where myosin IIA was enriched. In non-motile cells, myosin IIA was enriched in peripheral stress fibers with diphosphorylated RLCs, but myosin IIB was not. Our results suggest that myosin IIA may be highly activated by diphosphorylation of RLCs and primarily involved in cell migration.