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

Abstract

Several studies have shown that PKA-mediated phosphorylation of IP 3R1 at serines S 1588 and S 1755 enhances the receptor's ability to mobilize Ca 2+. In contrast, much less is known about whether Ca 2+ mobilization via IP 3R2 and IP 3R3 is regulated by PKA. We report here that IP 3R2 is only very weakly phosphorylated in response to PKA activation and is probably not a physiological substrate for this kinase. IP 3R3, however, is known to be phosphorylated by PKA at three sites (S 916, S 934, and S 1832) and, thus, we examined how phosphorylation of these sites affects Ca 2+ mobilization in DT40-3KO cells stably expressing either exogenous wild-type or mutant IP 3R3s; an antibody raised against phospho-serine 934 of IP 3R3 was used to demonstrate that the exogenous IP 3R3s are strongly phosphorylated in response to PKA activation. Surprisingly, our data show that IP 3R3-mediated Ca 2+ mobilization is unaffected by phosphorylation of S 916, S 934, and S 1832. In contrast, phosphorylation of exogenous IP 3R1 (monitored with an antibody against phospho-serine 1755) enhances Ca 2+ mobilization, indicating that DT40-3KO cells have the capacity to respond to phosphorylation of IP 3Rs. Overall, these data suggest that modification of Ca 2+ flux may not be the primary effect of IP 3R3 phosphorylation by PKA.