KRAP tethers IP(3) receptors to actin and licenses them to evoke cytosolic Ca(2+) signals

Regulation of IP(3) receptors (IP(3)Rs) by IP(3) and Ca(2+) allows regenerative Ca(2+) signals, the smallest being Ca(2+) puffs, which arise from coordinated openings of a few clustered IP(3)Rs. Cells express thousands of mostly mobile IP(3)Rs, yet Ca(2+) puffs occur at a few immobile IP(3)R clusters. By imaging cells with endogenous IP(3)Rs tagged with EGFP, we show that KRas-induced actin-interacting protein (KRAP) tethers IP(3)Rs to actin beneath the plasma membrane. Loss of KRAP abolishes Ca(2+) puffs and the global increases in cytosolic Ca(2+) concentration evoked by more intense stimulation. Over-expressing KRAP immobilizes additional IP(3)R clusters and results in more Ca(2+) puffs and larger global Ca(2+) signals. Endogenous KRAP determines which IP(3)Rs will respond: it tethers IP(3)R clusters to actin alongside sites where store-operated Ca(2+) entry occurs, licenses IP(3)Rs to evoke Ca(2+) puffs and global cytosolic Ca(2+) signals, implicates the actin cytoskeleton in IP(3)R regulation and may allow local activation of Ca(2+) entry.