Regulation of single inositol 1,4,5-trisphosphate receptor channel activity by protein kinase A phosphorylation.

Phosphorylation of inositol 1,4,5-trisphosphate receptors (InsP(3)R) by PKA represents an important, common route for regulation of Ca(2+) release. Following phosphorylation of the S2 splice variant of InsP(3)R-1 (S2-InsP-1), Ca(2+) release is markedly potentiated. In this study we utilize the plasma membrane (PM) expression of InsP(3)R-1 and phosphorylation state mutant InsP(3)R-1 to study how this regulation occurs at the single InsP(3)R-1 channel level. DT40-3KO cells stably expressing rat S2- InsP(3)R-1 were generated and studied in the whole-cell mode of the patch clamp technique. At hyperpolarized holding potentials, small numbers of unitary currents (average approximately 1.7 per cell) were observed which were dependent on InsP(3) and the presence of functional InsP(3)R-1, and regulated by both cytoplasmic Ca(2+) and ATP. Raising cAMP markedly enhanced the open probability (P(o)) of the InsP(3)R-1 and induced bursting activity, characterized by extended periods of rapid channel openings and subsequent prolonged refractory periods. The activity, as measured by the P(o) of the channel, of a non-phosphorylatable InsP(3)R-1 construct (Ser1589Ala/Ser1755Ala InsP(3)R-1) was markedly less than wild-type (WT) InsP(3)R-1 and right shifted some approximately 15-fold when the concentration dependency was compared to a phosphomimetic construct (Ser1589Glu/Ser1755Glu InsP(3)R-1). No change in conductance of the channel was observed. This shift in apparent InsP(3) sensitivity occurred without a change in InsP(3) binding or Ca(2+) dependency of activation or inactivation. Biophysical analysis indicated that channel activity can be described by three states: an open state, a long lived closed state which manifests itself as long interburst intervals, and a short-lived closed state. Bursting activity occurs as the channel shuttles rapidly between the open and short-lived closed state. The predominant effect of InsP(3)R-1 phosphorylation is to increase the likelihood of extended bursting activity and thus markedly augment Ca(2+) release. These analyses provide insight into the mechanism responsible for augmenting InsP(3)R-1 channel activity following phosphorylation and moreover should be generally useful for further detailed investigation of the biophysical properties of InsP(3)R.