Molecular cloning of mouse type 2 and type 3 inositol 1,4,5-trisphosphate receptors and identification of a novel type 2 receptor splice variant

Abstract

We isolated cDNAs encoding type 2 and type 3 inositol 1,4,5-trisphosphate (IP3) receptors (IP3R2 and IP3R3, respectively) from mouse lung and found a novel alternative splicing segment, SI m2, at 176-208 of IP3R2. The long form (IP3R2 SIm2+) was dominant, but the short form (IP3R2 SIm2-) was detected in all tissues examined. IP 3R2 SIm2- has neither IP3 binding activity nor Ca2+ releasing activity. In addition to its reticular distribution, IP3R2 SIm2+ is present in the form of clusters in the endoplasmic reticulum of resting COS-7 cells, and after ATP or Ca2+ ionophore stimulation, most of the IP3R2 SIm2+ is in clusters. IP3R3 is localized uniformly on the endoplasmic reticulum of resting cells and forms clusters after ATP or Ca2+ ionophore stimulation. IP3R2 SI m2- does not form clusters in either resting or stimulated cells. IP3 binding-deficient site-directed mutants of IP 3R2 SIm2+ and IP3R3 fail to form clusters, indicating that IP3 binding is involved in the cluster formation by these isoforms. Coespression of IP3R2 SI m2- prevents stimulus-induced IP3R clustering, suggesting that IP3R2 SIm2- functions as a negative coordinator of stimulusinduced IP3R clustering. Expression of IP3R2 SIm2- in CHO-K1 cells significantly reduced ATP-induced Ca2+ entry, but not Ca2+ release, suggesting that the novel splice variant of IP3R2 specifically influences the dynamics of the sustained phase of Ca2+ signals. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.