Phosphoinositides and calcium as regulators of cellular actin assembly and disassembly.

Cell locomotion and changes in cell structure initiated by the binding of extracellular ligands to transmembrane receptors are generally accompanied by a transient increase in cytoplasmic Ca2+ concentration, [Ca2+], and by the synthesis and hydrolysis of phosphorylated phosphatidylinositol lipids, or polyphosphoinositides (PPls). These two potential signals to the cyto­ skeleton are not necessarily the immediate consequence of receptor stimu­ lation, but are themselves modulated by upstream signals that include GTPases, protein kinases, ion channels, and probably other agents (reviewed in 118). Cell motility and changes in morphology also require spatially and temporally coordinated changes in the mechanical properties of the cell cortex, and these changes are brought about by transitions between gel (solid) and sol (liquid) states of the cytoskeleton (32, 119, 120). A link between Ca2+ and phospholipids, on the one hand, and changes in cell rigidity, on the other, was apparent enough to suggest nearly 40 years ago (52) that "the rigidity of the cortex depends on the fact that it contains