TIMAP (TGF-β-inhibited membrane-associated protein) is a regulatory subunit of protein phosphatase 1 (PP1). The N-terminal region contains a binding motif for the catalytic subunit of PP1 (PP1c) and a nuclear localization signal (NLS). Phosphorylation of TIMAP on Ser331, Ser333 and Ser337 side chains was shown to regulate the activity of the TIMAP-PP1c complex. Several studies, however, reported an additional side chain of TIMAP. Ser69 is located near to the PP1c binding motif and NLS, therefore, we hypothesized that the phosphorylation of this side chain perhaps may regulate the interaction between TIMAP and PP1c, or may affect the nuclear transport of TIMAP. Materials and Methods: To study the significance of Ser69 phosphorylation, GST-tagged or c-myc-tagged wild type, phosphomimic S69D and phosphonull S69A recombinant TIMAP proteins were expressed in bacteria or endothelial cells, respectively. Protein-protein interactions of the wild type or mutant forms of TIMAP were studied by pull-down and Western blot. Localization of TIMAP S69 mutants in pulmonary artery endothelial cells was detected by immunofluorescent staining and expression and localization of the recombinants were investigated by subcellular fractionation and Western blot. Results: Modifications of Ser69 of TIMAP had no effect on binding of PP1c, ERM or RACK1. However, S69D TIMAP showed enhanced membrane localization and an increased number of membrane protrusions were observed in the cells overexpressing this phosphomimic mutant. Furthermore, significantly faster wound healing and migration rate of the S69D mutant overexpressing cells were detected by endothelial barrier resistance measurements (ECIS). Specific interaction was shown between TIMAP and polo-like kinase 4 (PLK4), a potential kinase to phosphorylate Ser69. Conclusions: Altogether, our results indicate that Ser69 phosphorylation by PLK4 may evoke an enrichment of TIMAP in the plasma membrane region and may play an important role in endothelial cell migration without affecting the PP1c binding ability of TIMAP.
Keywords: Endothelium; TIMAP; cell migration; phosphatase; phosphorylation.