KRAS-induced actin-interacting protein regulates inositol 1,4,5-trisphosphate-receptor-mediated calcium release

KRAS-induced actin-interacting protein (KRAP) was originally characterized as a filamentous- actin-interacting protein. We have recently found that KRAP is an associated molecule with inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) and is responsible for the proper subcellular localization of IP₃R. Since it remains unknown whether KRAP regulates the IP₃R-mediated Ca²⁺ signaling, we herein examined the effects of KRAP on the IP₃R-mediated Ca²⁺ release by Ca²⁺ imagings in the cultured HEK293 or MCF7 cells. Reduction of KRAP protein by KRAP-specific siRNA diminishes ATP-induced Ca²⁺ release and the ATP-induced Ca²⁺ release is completely quenched by the pretreatment with the IP₃R inhibitor but not with the ryanodine receptor inhibitor, indicating that KRAP regulates IP₃R-mediated Ca²⁺ release. To further reveal mechanistic insights into the regulation of IP₃R-mediated Ca²⁺ release by KRAP, we examined the effects of the KRAP-knockdown on the releasable Ca²⁺ content of intracellular Ca²⁺ stores. Consequently, reduction of KRAP does not affect the amount of ionophore- or Ca²⁺-ATPase inhibitor-induced Ca²⁺ release in the HEK293 cells, indicating that releasable Ca²⁺ content of intracellular Ca²⁺ stores is not altered by KRAP. Thus, KRAP is involved in the proper regulation of IP₃R-mediated Ca²⁺ release.