Cell Biology Kinase as Scaffold

L. Bryan Ray

Science, Science Signaling, AAAS, Washington, DC 20005, USA

Phosphatidylinositol-4-phosphate 5-kinase (PIPKI) functions in cell adhesion and migration by catalyzing the synthesis of phosphatidylinositol-4,5-bisphosphate. Chao et al. demonstrate a second role of this enzyme in cultured human cells in which it controls actin polymerization and adhesion and migration through a function independent of its catalytic activity. Their results show that the PIPKI- isoform, but not the PIPKI-β isoform, serves as a scaffold protein that localizes the small guanosine triphosphatase Rac1, a key regulator of cell motility, to the plasma membrane at sites of integrin activation. Experiments in which cells were depleted of PIPKI with RNAi showed the enzyme to be required for adhesion-induced localization of Rac to the plasma membrane. This effect appeared to be independent of the enzyme’s lipid kinase activity because localization of Rac to the membrane was restored when a catalytically inactive mutant of the enzyme was expressed in the cells. PIPKI- directly interacted with Rac1, and mistargeting PIPKI- to the mitochondria caused Rac1 to be mislocalized as well. In wound closure assays, loss of PIPKI- led to decreased cell motility and a tendency to wander randomly rather than migrate in one direction. Loss of PIPKI- also disrupted membrane ruffling and organization of the actin cytoskeleton. The defects in cells lacking PIPKI- were diminished if cells were engineered to express a modified form of Rac1 targeted to the plasma membrane. Thus, although PIPKI- functions enzymatically to guide other small GTPases to the plasma membrane by generating PI4,5P2 and PI3,4,5P3 (to which the GTPases bind), it has another more direct mode of regulating Rac1.

W.-T. Chao, A. C. Dacquinag, F. Ashcroft, J. Kunz, Type I PIPK- regulates directed cell migration by modulating Rac1 plasma membrane targeting and activation. J. Cell Biol. 190, 247–262 (2010). [Abstract] [Full Text]