Lipid Products of Phosphoinositide 3-Kinase Bind Human Profilin with High Affinity

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Abstract
To gain insight into the physiological function of phosphoinositide 3-kinase (PI 3-kinase) lipid products, this study examines the interactions of the D-3 phosphoinositides with profilin and the consequent effects on actin dynamics and phosphoinositide turnover. Profilin, a ubiquitous actin-regulating protein, plays a putative role in regulating actin assembly and PLC-γ1 signaling in light of its unique interactions with actin and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. Here we raise evidence that the affinity of profilin with the D-3 phosphoinositides is substantially higher than that of PtdIns(4,5)P2. The dissociation constants (Kd) are estimated to be 1.1 μM, 5.7 μM, and 11 μM for phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P2], phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3], and PtdIns(4,5)P2, respectively. Spectroscopic data show that while all these phosphoinositides alter the tryptophan fluorescence of profilin in a similar fashion, the respective conformational effect on profilin is vastly different. Based on CD data, the α-helical contents of profilin in the presence of 8 molar equiv of PtdIns(4,5)P2, PtdIns(3,4,5)P3, and PtdIns(3,4)P2 are 17.4%, 11.5%, and 1.4%, respectively, vis-à-vis 9.4% for profilin alone. In contrast, no appreciable change in the fluorescence and CD spectra is observed when related inositol phosphates such as Ins(1,4,5)P3, Ins(1,3,4,5)P4, or Ins(1,3,4)P3 at comparable concentrations are tested. Evidence suggests that this differential recognition bears functional significance concerning the intricate roles of profilin and inositol lipids in modulating actin polymerization and PtdIns(4,5)P2 turnover. The relative potency of individual phosphoinositides in offsetting the inhibitory effect of profilin on actin assembly is PtdIns(3,4)P2 > PtdIns(3,4,5)P3 > PtdIns(4,5)P2, consistent with their relative binding affinity with profilin. Moreover, the inhibitory effect of profilin on PLC-γ1-mediated PtdIns(4,5)P2 hydrolysis is overcome by PtdIns(3,4)P2 and PtdIns(3,4,5)P3 through a combined effect of PLC-γ1 activation and preferential profilin binding. This D-3 phosphoinositide-mediated regulation may represent a new mechanism for controlling PtdIns(4,5)P2 turnover by PLC-γ1.