Gelsolin, an actin and phosphoinositide binding protein, was photoaffinity labeled using a variety of benzophenone-containing phosphoinositide polyphosphate analogues. The N-terminal half and the C-terminal half of gelsolin showed synergy in the binding of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. Competitive displacement experiments with dibutyryl, dioctanoyl, or dipalmitoyl derivatives of PtdIns(4,5)P(2) suggested that, in addition to the inositol headgroup, a diacylglyceryl moiety was important for binding; these analogues also inhibited the gelsolin-severing activity of F-actin. In addition to the previously identified PtdIns(4,5)P2 binding site in the N-terminal half of gelsolin, a new binding site was identified in the C-terminal half by mapping the photocovalently modified peptide fragments. Moreover, increasing concentrations of Ca(2+) decreased the binding of the photolabile analogues to the C-terminal phosphoinositide binding site on gelsolin. A molecular model of the binding of PtdIns(4,5)P2 within two folded repeats of gelsolin has been calculated using these data.