80K-H interacts with inositol 1,4,5-trisphosphate (IP ₃) receptors and regulates IP ₃-induced calcium release activity

Inositol 1,4,5-trisphosphate receptors (IP ₃Rs) are intracellular channel proteins that mediate calcium (Ca ²⁺) release from the endoplasmic reticulum, and they are involved in many biological processes (e.g. fertilization, secretion, and synaptic plasticity). Recent reports show that IP ₃R activity is strictly regulated by several interacting molecules (e.g. IP ₃R binding protein released with inositol 1,4,5-trisphosphate, huntingtin, presenilin, DANGER, and cytochrome c), and perturbation of this regulation causes intracellular Ca ²⁺ elevation leading to several diseases (e.g. Huntington disease and Alzheimer disease). In this study, we identified protein kinase C substrate 80K-H (80K-H) to be a novel molecule interacting with the COOH-terminal tail of IP ₃Rs by yeast two-hybrid screening. 80K-H directly interacted with IP ₃R type 1 (IP ₃R1) in vitro and co-immunoprecipitated with IP ₃R1 in cell lysates. Immunocytochemical and immunohistochemical staining revealed that 80K-H colocalized with IP ₃R1 in COS-7 cells and in hippocampal neurons. We also showed that the purified recombinant 80K-H protein directly enhanced IP ₃-induced Ca ²⁺ release activity by a Ca ²⁺ release assay using mouse cerebellar microsomes. Furthermore 80K-H was found to regulate ATP-induced Ca ²⁺ release in living cells. Thus, our findings suggest that 80K-H is a novel regulator of IP ₃R activity, and it may contribute to neuronal functions.