Differential recognition of phosphoinositides by actin regulating proteins and its physiological implications

In an effort to understand the intricate relationship between phosphoinositide 3-kinase (PI 3-kinase) and actin assembly, we examined the differential interaction between phosphoinositides and actin-binding proteins (ABPs). The affinities for inositol lipids of three important ABPs (profilin, gelsolin, and CapG) are assessed by gel filtration and fluorescent titration. These analyses indicate that profilin displays preferential binding to the D-3 phosphoinositides over PtdIns(4,5)P₂, whereas gelsolin and CapG favor PtdIns(4,5)P₂. Also noteworthy is that the binding of gelsolin and CapG to PtdIns(4,5)P₂ is Ca²⁺-dependent. The binding affinities in the presence of Ca²⁺ are 7.5-and 2.4-fold higher than that without Ca²⁺ for gelsolin and CapG, respectively. Furthermore, the present data support the notion that these ABPs provide a link between actin dynamics and PtdIns(4,5)P₂-dependent signaling pathways. The regulatory effect of ABPs on actin assembly is altered by phosphoinositides with potencies conforming to the respective binding affinities. Meanwhile, ABPs exert an inhibitory effect on the activity of PtdIns(4,5)P₂-utilizing enzymes in part by controlling PtdIns(4,5)P₂ availability. This inhibition may represent a negative feedback control of PI 3-kinase. Taken together, a working model correlating PI 3-kinase activation and the regulation of actin assembly by profilin and gelsolin is proposed.