Differential regulation of types-1 and -3 inositol trisphosphate receptors by cytosolic Ca2+.
Biphasic regulation of inositol trisphosphate (IP3)-stimulated Ca2+ mobilization by cytosolic Ca2+ is believed to contribute to regenerative intracellular Ca2+ signals. Since cells typically express several IP3 receptor isoforms and the effects of cytosolic Ca2+ are not mediated by a single mechanism, it is important to resolve the properties of each receptor subtype. Full-length rat types-1 and -3 IP3 receptors were expressed in insect Sf9 cells at levels 10-40-fold higher than the endogenous receptors. The expressed receptors were glycosylated and assembled into tetramers, and binding of [3H]IP3 to each subtype was regulated by cytosolic Ca2+. The effects of increased [Ca2+] on native cerebellar and type-1 receptors expressed in Sf9 cells were indistinguishable. A maximally effective increase in [Ca2+] reversibly inhibited [3H]IP3 binding by approx. 50% by decreasing the number of IP3-binding sites (Bmax) without affecting their affinity for IP3. The effects of Ca2+ on type-3 receptors were more complex: increasing [Ca2+] first stimulated [3H]IP3 binding by increasing Bmax, and then inhibited it by causing a substantial decrease in the affinity of the receptor for IP3. The different effects of Ca2+ on the receptor subtypes were not a consequence of limitations in the availability of accessory proteins or of artifactual effects of Ca2+ on membrane structure. We conclude that Ca2+ can inhibit IP3 binding to types-1 and -3 IP3 receptors although by different mechanisms, and that IP3 binding to type-3 receptors is stimulated at intermediate [Ca2+]. A consequence of these differences is that, at resting cytosolic [Ca2+], type-3 receptors are more sensitive than type-1 receptors to IP3, but the situation reverses at higher cytosolic [Ca2+]. Such differences may be important in generating the spatially and temporally complex changes in cytosolic [Ca2+] evoked by receptors linked to IP3 formation.
Cardy
T J
TJ
Department of Pharmacology, Tennis Court Road, Cambridge CB2 1QJ, U. K.
Traynor
D
D
Taylor
C W
CW
D000595:Amino Acid Sequence; D000818:Animals; D016367:Baculoviridae / Q000235:genetics; D001665:Binding Sites; D001667:Binding, Competitive; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D003600:Cytosol / Q000378:metabolism; D006031:Glycosylation; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008565:Membrane Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D011487:Protein Conformation; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D018411:Spodoptera
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008565:Membrane Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D011994:Recombinant Proteins; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium