Temperature-dependence and conformational basis of inositol 1,4,5-trisphosphate receptor regulated by Ca(2+)

The inositol 1,4,5-trisphosphate (InsP(3)) receptor was purified from bovine cerebellum and reconstituted in liposomes composed of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) (1:1) successfully. No effect of Ca(2+) concentration on [(3)H]-InsP(3) binding to unreconstituted InsP(3) receptor could be observed either at 4 degrees C or at 25 degrees C, whereas the effect of [Ca(2+)] on reconstituted InsP(3) receptor depended on the temperature. The Ca(2+) concentration outside the proteolipsome ([Ca(2+)](o)) had no detectable effect on InsP(3) binding to InsP(3) receptor at 4 degrees C. In contrast, with increase of [Ca(2+)](o) from 0 to 100 nmol/L at 25 degrees C, the InsP(3) binding activity increased gradually. Then the InsP(3) binding activity was decreased drastically at higher [Ca(2+)](o) and inhibited entirely at 50 mumol/L [Ca(2+)](o). Conformational studies on intrinsic fluorescence of the reconstituted InsP(3) receptor and its quenching by KI and HB indicated that the global conformation of reconstituted InsP(3) receptor could not be affected by [Ca(2+)](o) at 4 degrees C. While at 25 degrees C, the effects of 10 mumol/L [Ca(2+)](o) on global, membrane and cytoplasmic conformation of the reconstituted InsP(3) receptor were different significantly from that of 100 nmol/L [Ca(2+)](o).