ISOPRENYLATED HUMAN BRAIN-TYPE-I INOSITOL 1,4,5-TRISPHOSPHATE 5-PHOSPHATASE CONTROLS CA2-HAMSTER OVARY CELLS( OSCILLATIONS INDUCED BY ATP IN CHINESE)

D-myo-Inositol 1,4,5-trisphosphate (InsP(3)) 5-phosphatase and 3-kinas e are thought to be critical regulatory enzymes in the control of InsP (3) and Ca2+ signaling, In brain and many other cells, type I InsP(3) B-phosphatase is the major phosphatase that dephosphorylates InsP(3) a nd D-myo-inositol 1,3,4,5-tetrakisphosphate. The type I B-phosphatase appears to be associated with the particulate fraction of cell homogen ates, Molecular cloning of the human brain enzyme identifies a C-termi nal farnesylation site CVVQ. Post-translational modification of this e nzyme promotes membrane interactions and changes in specific activity, We have now compared the cytosolic Ca2+ ([Ca2+](i)) responses induced by ATP, thapsigargin, and ionomycin in Chinese hamster ovary (CHO-K1) cells transfected with the intact InsP(3) B-phosphatase and with a mu tant in which the C-terminal cysteine cannot be farnesylated, [Ca2+](i ) was also measured in cells transfected with an InsP(3) 3-kinase cons truct encoding the A isoform, The Ca2+ oscillations detected in the pr esence of 1 mu M ATP in control cells were totally lost in 87.5% of in tact (farnesylated) InsP(3) B-phosphatase-transfected cells, while suc h a loss occurred in only 1.1% of the mutant InsP(3) 5-phosphatase-tra nsfected cells, All cells overexpressing the InsP(3) S-kinase also res ponded with an oscillatory pattern, However, in contrast to control ce lls, the [Ca2+](i) returned to base-line levels in between a couple of oscillations, The [Ca2+](i) responses to thapsigarin and ionomycin we re identical for all cells, The four cell clones compared in this stud y also behaved similarly with respect to capacitative Ca2+ entry, In p ermeabilized cells, no differences in extent of InsP(3)-induced Ca2+ r elease nor in the threshold for InsP(3) action were observed among the four clones and no differences in the expression levels of the variou s InsP(3) receptor isoforms could be shown between the clones, Our dat a support the contention that the ATP-induced increase in InsP(3) conc entration in transfected CHO-KI cells is essentially restricted to the site of its production near the plasma membrane, where it can be meta bolized by the type I InsP(3) B-phosphatase. This enzyme directly cont rols the [Ca2+](i) response and the Ca2+ oscillations in intact cells.