Analysis of the PRK family of Rho GTPase effector proteins in the context of cell cycle progression and epithelial morphogenesis

Abstract

Rho GTPases are signal transduction proteins that affect a wide variety of cellular processes through the interaction with effector proteins. The protein kinase C related kinases (PRK) are a family of Rho GTPase effectors composed of three members: PRK1, PRK2 and PRK3. PRKs have a catalytic domain homologous to protein kinase C and a regulatory domain that interacts with Rho GTPases. Depletion of PRK2 by siRNA in mammalian cells has been shown to affect cell cycle progression. The way PRK2 function is regulated during the cell cycle is, however, not well understood and is the main focus of the present work. It was shown that PRK2 is phosphorylated in mitosis in a way that gives rise to a mobility shift when subjected to SDS-PAGE gel electrophoresis. Here we were able to show that PRK2 is phosphorylated when cells are in prometaphase and metaphase and is dephosphorylated as cells progress to cytokinesis. Using an in vitro kinase assay to compare the activities of mitotically phosphorylated PRK2 and interphase PRK2, we observed that PRK2 is slightly less active when phosphorylated. We have also identified two sites in PRK2 that are specifically phosphorylated in mitosis. PRK2 localizes at the cleavage furrow and around the midbody during cytokinesis. The HR1 domain, which binds Rho and Rac, is required for PRK2 localization during cytokinesis. Rho is an important regulator of cytokinesis that shows a similar localization pattern to PRK2 and we propose that Rho mediates PRK2 localization during cytokinesis. While analyzing the localization of PRK2 during cytokinesis, we realized that PRK2 also localizes at sites of cell-cell contact that form between daughter cells and that the HR1 domain is also required for this localization. In addition, PRK2 localizes at apical junctions in polarized epithelial cells. These results suggest a new role for PRK2 during epithelia morphogenesis. We propose a functional link between the localization of PRK2 at the cleavage furrow/midbody and at apical junctions. We propose that junctional/polarity proteins are recruited to the cleavage furrow or around the midbody during cytokinesis to facilitate junction formation between daughter cells.