THE PROPERTIES OF A SUBTYPE OF THE INOSITOL 1,4,5-TRISPHOSPHATE RECEPTOR RESULTING FROM ALTERNATIVE SPLICING OF THE MESSENGER-RNA IN THE LIGAND-BINDING DOMAIN

Subtypes of the type-1 inositol 1,4,5-trisphosphate (InsP(3)) receptor differ at the mRNA level in two small variably spliced segments. One segment (SI) encodes for a sequence within the InsP(3)-binding domain, thus its presence or absence could affect the functions of the recept or. We have used anti-peptide antibodies to confirm the existence of d ifferent subtypes of the InsP(3) receptor (InsP(3)R) protein. The anti body against residues 322-332, within the SI region, recognized a 260 kDa polypeptide in membranes prepared from rat cerebellum or cerebral cortex. The cerebellum contained a few percent of the InsP(3)R protein having the SI region, whereas the cerebral cortex contained a high pr oportion of receptors with the SI region. These two tissues were repre sentative of both isoforms, SI- or SI+, and displayed the same [H-3]In sP(3)-binding characteristics. Thus, the SI region was not involved in the basic properties of the receptor. Deletion of the peptide 316-352 containing the SI segment greatly reduced InsP(3) binding [Miyawaki, Furuichi, Ryou, Yoshikawa, Nakagawa, Saitoh and Mikoshiba (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 4911-4915]. The antibodies against the SI region or against residues 337-349 did not modify the binding of [H-3 ]InsP(3) in the cortical membranes rich in the SI+ isoform or in cereb ellar membranes. These results suggested that the SI region was not pa rt of the binding site. The subcellular distribution of these two isof orms was then investigated in rat liver. The two isoforms were identif ied in different membrane fractions and they followed the same subcell ular distribution. We suggest that the domain with the SI region may b e involved in a function other than InsP(3)-induced Ca2+ release.