Calcium mobilization via type III inositol 1,4,5-trisphosphate receptors is not altered by PKA-mediated phosphorylation of serines 916, 934, and 1832.

Several studies have shown that PKA-mediated phosphorylation of IP3R1 at serines S1588 and S1755 enhances the receptor's ability to mobilize Ca2+. In contrast, much less is known about whether Ca2+ mobilization via IP3R2 and IP3R3 is regulated by PKA. We report here that IP3R2 is only very weakly phosphorylated in response to PKA activation and is probably not a physiological substrate for this kinase. IP3R3, however, is known to be phosphorylated by PKA at three sites (S916, S934, and S1832) and, thus, we examined how phosphorylation of these sites affects Ca2+ mobilization in DT40-3KO cells stably expressing either exogenous wild-type or mutant IP3R3s; an antibody raised against phospho-serine 934 of IP3R3 was used to demonstrate that the exogenous IP3R3s are strongly phosphorylated in response to PKA activation. Surprisingly, our data show that IP3R3-mediated Ca2+ mobilization is unaffected by phosphorylation of S916, S934, and S1832. In contrast, phosphorylation of exogenous IP3R1 (monitored with an antibody against phospho-serine 1755) enhances Ca2+ mobilization, indicating that DT40-3KO cells have the capacity to respond to phosphorylation of IP3Rs. Overall, these data suggest that modification of Ca2+ flux may not be the primary effect of IP3R3 phosphorylation by PKA.