Inositol 1,4,5-trisphosphate (IP₃) receptors belong to a family of intracellular Ca²⁺ channels that mediate release of Ca²⁺ from intracellular stores and thereby generate increases in cytosolic [Ca²⁺] that are often spatially complex. IP₃ receptors are expressed largely within the membranes of the endoplasmic reticulum, but they are also found in the nuclear envelope, Golgi, secretory vesicles and perhaps in the plasma membrane. Targeting of IP₃ receptors to appropriate membranes is likely to be important in determining the patterns of Ca²⁺ release evoked by extracellular stimuli. A previous study in Xenopus oocytes suggested that the six transmembrane regions (TMR) together with the carboxyl terminal of the type 1 IP₃ receptor was sufficient to ensure its retention within the endoplasmic reticulum (Sayers et al. 1997). Recent work in mammalian cells further supports a role for TMRs in efficient targeting of IP₃ receptors to intracellular membranes (Galvan et al. 1999).

We use confocal imaging of mouse insulinoma cells (MIN6) and COS-7 cells to determine the localization of both the endoplasmic reticulum (using an antibody to an ER-resident protein, BiP) and expressed fragments of the type 1 IP₃ receptor tagged with green fluorescent protein (GFP). Results are from at least six independent transfections for all constructs. We confirm that a protein containing all six TMR and the carboxyl terminal of the receptor is targeted to the ER. The targeting was unaffected by deletion of the carboxyl terminal. Fusion proteins that included only the first four TMR, the first two TMR or the last two TMR of the IP₃ receptor together with the linking regions were also targeted to the ER. We conclude that both TMR1Ð2 and TMR5Ð6 are independently capable of targeting the type 1 IP₃ receptor to the ER. Targeting of IP₃ receptors to the ER appears to be a redundant process mediated by non-sequence-specific properties of the TMRs and does not require the carboxyl terminal cytoplasmic tail.

This work was supported by a BBSRC Studentship (A.K.-T.P.), a Royal Society Dorothy Hodgkin Fellowship (F.V.G.) and The Wellcome Trust.