The Role of Phosphatidic Acid in PIPKI-regulated Actin Reorganization

Abstract

Production and breakdown of certain phosphoinositides plays key roles in a variety of cellular processes. Among these, phosphatidylinositol-4, 5-phosphate (PIP ₂) is well known for its critical functions in membrane trafficking, cytoskeletal organization, and signal transduction. Type I phosphatidylinositol-4-phosphate 5-kinase (PIPKI) is the main enzyme responsible for the synthesis of PIP₂. Although PIPKI activity is regulated by small G proteins and phosphatidic acid (PA), the contribution of these upstream regulators in actin cytoskeletal reorganization remains unclear. The binding region of some other PA-regulated proteins has been identified. From these, it appears that binding may either be direct or electrostatic in nature. I have identified basic residues in the proposed membrane-binding region of PIPKIΓ that are required for membrane translocation, actin reorganization, and stimulation by PA. I also demonstrate here that the direct binding of PIPKI to PA through these residues is required for these important functions.