MUSCLE-SPECIFIC MESSENGER-RNA ISOFORM ENCODES A PROTEIN COMPOSED MAINLY OF THE N-TERMINAL-175 RESIDUES OF TYPE-2 INS(1,4,5)P-3 RECEPTOR

We have found a novel isoform of the mouse type 2 Ins(1,4,5)P-3 recept or [Ins(1,4,5)P3R] mRNA by reverse transcriptase-mediated PCR analysis . The novel isoform, which was expressed specifically in skeletal musc le and heart, was generated by the inclusion of a novel exon. As this exon contains a stop codon, the isoform encodes a putative protein (de signated TIPR)consisting of 175 acid residues of the type 2 Ins(1,4,5) P3R and the following six residues derived from this exon.We transfect ed the cDNA of this isoform into COS-7 cells; these cells expressed a 24 kDa protein that was recognized by an antibody against TIPR produce d in Escherichia coli. The isoform encoding TIPR was also found in hum an skeletal muscle and heart. The N-terminal region of Ins(1,4,5)P3R i s suggested to have a role in ligand binding and to interact with the C-terminal channel domain of Ins(1,4,5)P3R itself. TIPR might regulate the Ins(1,4,5)P-3 signal pathway in both muscles.