The immunophilin FKBP12 is one of the most abundant and conserved proteins in biology. It is the primary receptor for the immunosuppressant actions of the drug FK506 in whose presence FKBP12 binds to and inhibits calcineurin, disrupting interleukin formation in lymphocytes. The physiologic functions of FKBP12 are less clear, although the protein has been demonstrated to physiologically interact with the inositol 1,4,5-trisphosphate receptor (IP3R), the ryanodine receptor, and the type 1 transforming growth factor beta receptor. We now report that FKBP12 binds the IP3R at residues 1400-1401, a leucyl-prolyl dipeptide epitope that structurally resembles FK506. We further demonstrate that binding to IP3R at this site enables FKBP12 to interact with calcineurin, presumably to anchor the phosphatase to IP3R and modulate the receptor's phosphorylation status. We propose that FK506 promotes an FKBP12-calcineurin interaction by mimicking structurally similar dipeptide epitopes present within proteins that use FKBP12 to anchor calcineurin to the appropriate physiologic substrates.

The immunophilin FK506 binding protein 12 (FKBP12) is associated with and modulates the ryanodine receptor calcium release channel of skeletal muscle. Ryanodine receptor has amino acid homology and functional similarity with another intracellular Ca2+ release channel, the inositol 1,4,5-trisphosphate receptor (IP3R). In the present study we show that highly purified preparations of IP3R contain FKBP12. The complex of these two proteins is disrupted by the immunosuppressants FK506 and rapamycin, both of which are known to bind FKBP12 with high affinity. Disrupting the IP3R-FKBP12 interaction increases Ca2+ flux through IP3R, an effect that is reversed by added FKBP12. FKBP12 appears to be physiologically linked to IP3R, regulating its Ca2+ conductance.

Release of intracellular Ca2+ by inositol 1,4,5-trisphosphate (InsP3) occurs through specific receptor proteins which are ligand-activated Ca2+ channels. Changes in intracellular Ca2+ regulate many cellular functions. This Ca2+ release is a discontinuous quantal process in which successive increments of InsP3 transiently release precise amounts of Ca2+ (refs 4-6). Possible explanations of quantal Ca2+ release have included rapid degradation of InsP3, reciprocity of Ca2+ release and sequestration, desensitization of InsP3 receptors, or actions of InsP3 on discrete compartments of Ca2+ with variable sensitivity to InsP3 (ref. 4). We successfully reconstituted InsP3-induced Ca2+ flux in vesicles containing only purified InsP3 receptor protein. The reconstituted vesicles retain the regulatory features of the InsP3 receptor, including phosphorylation sites and modulation of Ca2+ release by adenine nucleotides. Using these reconstituted vesicles, we show here that quantal flux of Ca2+ elicited by InsP3 is a fundamental property of its receptor.

Inositol 1,4,5-trisphosphate (IP3) releases internal stores of calcium by binding to a specific membrane receptor which includes both the IP3 recognition site as well as the associated calcium channel. The IP3 receptor is regulated by ATP, calcium, and phosphorylation by protein kinase A, protein kinase C, and calcium/calmodulin-dependent protein kinase II. Its cDNA sequence predicts at least two consensus sequences where nucleotides might bind, and direct binding of ATP to the IP3 receptor has been demonstrated. In the present study, we demonstrate autophosphorylation of the purified and reconstituted IP3 receptor on serine and find serine protein kinase activity of the IP3 receptor toward a specific peptide substrate. Several independent purification procedures do not separate the IP3 receptor protein from the phosphorylating activity, and many different protein kinase activators and inhibitors do not identify protein kinases as contaminants. Also, renaturation experiments reveal autophosphorylation of the monomeric receptor on polyvinylidene difluoride membranes.

IP3 activates intracellular calcium release by binding to an intracellular ligand gated calcium permeable channel which has been shown to be regulated by protein kinase A phosphorylation. Two consensus sequences for protein kinase A phosphorylation are predicted by the recently isolated cDNA of the mouse and rat. In the present study we have isolated and sequenced the two peptides in the rat IP3 receptor which are phosphorylated by protein kinase A and demonstrate protein kinase A phosphorylation on S-1755 and S-1589.

We have previously demonstrated that the inositol 1,4,5-trisphosphate (IP3) receptor is phosphorylated by cyclic AMP-dependent protein kinase (PKA). In the present study, phosphorylation of IP3 receptors has been examined with purified receptor protein reconstituted in liposomes to remove detergent that can inhibit protein kinases. The IP3 receptor is stoichiometrically phosphorylated by protein kinase C (PKC) and Ca2+ calmodulin-dependent protein kinase II (CaM kinase II) as well as by PKA. Phosphorylation by the three enzymes is additive and involves different peptide sequences. Phosphorylation by PKC, which is stimulated by Ca2+ and diacylglycerol, and by CaM kinase II, which requires Ca2+, provides means whereby Ca2+ and diacylglycerol, formed during inositol phospholipid turnover, may regulate IP3 receptor physiology.