MOLECULAR-INTERACTIONS OF ENDOGENOUS D-MYO-INOSITOL PHOSPHATES WITH THE INTRACELLULAR D-MYO-INOSITOL 1,4,5-TRISPHOSPHATE RECOGNITION SITE

A systematic effort was made to elucidate the mode of recognition at the inositol 1,4,5-trisphosphate-specific receptor. Eleven D-myo-inositol phosphates were synthesized and tested for Ca2+ mobilizing and receptor-binding activities, which included Ins(1,3,4,5,6)P-5, Ins(1,2,5,6)P-4, Ins(1,3,4,5)P-4, Ins(1,3,4,6)P-4, Ins(1,4,5,6)P-4, Ins(3,4,5,6)P-4, Ins(1,3,4)P-3, Ins(1,4,5)P-3, Ins(1,5,6)P-3, Ins(1,4)P-2, and Ins(4,5)P-2. Of these, Ins(1,4,5)P-3, Ins(1,3,4,6)P-4, Ins(1,3,4,5)P-4, Ins(1,4,5,6)P-4, and Ins(4,5)P-2 were able to elicit Ca2+ release from rat brain microsomes. Binding experiments suggest that the ability of these polyphosphates to effect Ca2+ mobilization arises from interactions with the Ins(1,4,5)P-3-specific receptor. Accordingly, a model accounting for the ligand recognition is proposed. The Ins(1,4,5)P-3-binding site is presumably composed of two domains. The anchoring domain binds the 4,5-bisphosphate 6-hydroxy motif. Disruption of this structural feature abolishes the agonist activity. The auxiliary domain exerts long-range interactions with the 1-phosphate, thus enhancing the binding affinity. The stereochemical requirement for this electrostatic interaction is, however, less stringent. Evidence suggests that Ca2+-mobilizing inositol phosphates are able to effect productive binding by assuming conformations displaying or mimicking these essential structural features.