The type III inositol 1,4,5-trisphosphate receptor is phosphorylated by cAMP-dependent protein kinase at three sites

IP₃ (inositol 1,4,5-trisphosphate) receptors form tetrameric, IP₃-gated Ca²⁺ channels in endoplasmic reticulum membranes, and are substrates for several kinases, including PKA (cAMP-dependent protein kinase). Activation of PKA has been reported to either enhance or inhibit type III IP₃ receptor Ca²⁺-channel activity, but, as yet, the sites of phosphorylation remain unknown. Here, we reveal that PKA phosphorylates the type III IP₃ receptor at Ser⁹¹⁶, Ser⁹³⁴ and Ser¹⁸³², and that, intriguingly, each site is located close to a putative surface-exposed peptide loop. Furthermore, we demonstrate that Ser⁹³⁴ is considerably more susceptible to PKA-dependent phoshorylation than either Ser⁹¹⁶ or Ser¹⁸³². These findings define the sites at which the type III IP₃ receptor is phosphorylated by PKA, and provide the basis for exploring the functional consequences of this regulatory event.