Differential recognition of phosphatidylinositol polyphosphates by profilin

Molecular interactions of profilin with various phosphoinositides including PtdIns(4,5)P₂, PtdIns(3,4,5)P₃, and PtdIns(3,4)P₂ were examined. Gel filtration and fluorescence titration analyses indicated that the molecular stoichiometry for each profilin and dissociation constants were: PtdIns(3,4)P₂: 5.5, 0.3 μM; PtdIns(3,4,5)P₃: 6, 1 μM ; PtdIns(4,5)P₂: 10, 3.6 μM. Moreover, circular dichroism spectroscopy showed that these inositol lipids induced vastly different conformational changes in profilin. The α-helical contents of profilin in the presence of PtdIns(4,5)P₂, PtdIns(3,4,5)P₃, and PtdIns(3,4)P₂ were 17.4%, 11.5%, and 1.4%, respectively, vis-a-vis 9.4% for profilin alone. Based on these spectroscopic data, a working model of phosphoinositide binding to profilin is proposed. This differential recognition bears two folds of physiological implications. First, it provides a putative link between phosphoinositide 3-kinase activation and actin assembly. Secondly, the high affinity of D-3 phosphoinositides with profilin may increase the exposure of PtdIns(4,5)P₂ to PLC, thus regulating the production of Ins(1,4,5)P₃.