cAMP‐dependent protein kinase phosphorylates the type III inositol 1,4,5‐trisphosphate receptor at 3 sites

We used a mutagenic approach to define the sites at which the type III inositol 1,4,5‐trisphosphate receptor (IP 3 R3) is phosphorylated by cAMP‐dependent protein kinase (PKA). Wild type and mutant IP 3 R3s were expressed in HEK293 cells and endogenous PKA was activated by stimulation with 10μM forskolin or 1μM prostaglandin E 1 (PGE 1 ). We mutated 51 serine or threonine residues at sites resembling the PKA consensus sequences R‐R/K‐X‐S/T, R‐X 2 ‐S/T, or R‐X‐S/T, and found that 3 serine residues (S 916 , S 934 , and S 1832 ) contributed to IP 3 R3 phosphorylation. Furthermore, we determined that PKA preferentially phosphorylates S 934 ; ~50% of total IP 3 R3 phosphorylation occurs at S 934 , while S 916 and S 1832 each contribute ~25%. Studies are currently underway to determine the functional effects of this phosphorylation on IP 3 R3‐mediated Ca 2+ signaling at the single cell level. Further, we have developed an antibody specific to IP 3 R3 phosphorylated at S 934 , allowing us to study the prevalence of IP 3 R3 phosphorylation in various tissues. Taken together, these studies will elucidate the functional consequences of IP 3 R3 phosphorylation. We would like to thank AHA (0256225T) and NIH (DK49194) for financial support.