Type I phosphatidylinositol 4-phosphate 5-kinase homo- and heterodimerization determines its membrane localization and activity

Type I phosphatidylinositol 4-phosphate 5-kinases (PIP5KIs; alpha, beta, and gamma) are a family of isoenzymes that produce phosphatidylinositol 4,5-bisphosphate [PI(4,5)P-2] using phosphatidylinositol 4-phosphate as substrate. Their structural homology with the class II lipid kinases [type II phosphatidylinositol 5-phosphate 4-kinase (PIP4KII)] suggests that PIP5KI dimerizes, although this has not been formally demonstrated. Neither the hypothetical structural dimerization determinants nor the functional consequences of dimerization have been studied. Here, we used Forster resonance energy transfer, coprecipitation, and ELISA to show that PIP5KI beta forms homo- and heterodimers with PIP5KI gamma_i2 in vitro and in live human cells. Dimerization appears to be a general phenomenon for PIP5KI isoenzymes because PIP5KI beta/PIP5KI alpha heterodimers were also detected by mass spectrometry. Dimerization was independent of actin cytoskeleton remodeling and was also observed using purified proteins. Mutagenesis studies of PIP5KI beta located the dimerization motif at the N terminus, in a region homologous to that implicated in PIP4KII dimerization. PIP5KI beta mutants whose dimerization was impaired showed a severe decrease in PI(4,5)P-2 production and plasma membrane delocalization, although their association to lipid monolayers was unaltered. Our results identify dimerization as an integral feature of PIP5K proteins and a central determinant of their enzyme activity.