Targeting of inositol 1,4,5-trisphosphate receptor to the endoplasmic reticulum by its first transmembrane domain

Targeting of IP(3)R (inositol 1,4,5-trisphosphate receptors) to membranes of the ER (endoplasmic reticulum) and their retention within ER or trafficking to other membranes underlies their ability to generate spatially organized Ca(2+) signals. N-terminal fragments of IP(3)R1 (type 1 IP(3)R) were tagged with enhanced green fluorescent protein, expressed in COS-7 cells and their distribution was determined by confocal microscopy and subcellular fractionation. Localization of IP(3)R1 in the ER requires translation of between 26 and 34 residues beyond the end of the first transmembrane domain (TMD1), a region that includes TMD2 (second transmembrane domain). Replacement of these post-TMD1 residues with unrelated sequences of similar length (24–36 residues) partially mimicked the native residues. We conclude that for IP(3)R approx. 30 residues after TMD1 must be translated to allow a signal sequence within TMD1 to be extruded from the ribosome and mediate co-translational targeting to the ER. Hydrophobic residues within TMD1 and TMD2 then ensure stable association with the ER membrane.