Analysis of structural signals conferring localisation of pig OST48 to the endoplasmic reticulum

Pig liver oligosaccharyltransferase (OST) is a heterooligomeric protein complex responsible for the cotranslational transfer of GlcNAc(2)-Man(9)-Glc(3) from Dol-PP onto specific asparagine residues in the nascent polypeptide. OST48, one of the catalytic subunits in this complex, exerts a typical type I membrane topology, containing a large luminal domain, a hydrophobic transmembrane domain and a short cytosolic peptide tail. Because OST48 is found within the endoplasmic reticulum (ER) when overexpressed in COS-1 cells, we carried out experiments to identify structural signals potentially capable of directing ER-targeting, using OST48 mutants and hybrid proteins consisting of individual OST48 domains and Man(9)-mannosidase. Immunofluorescence microscopy showed that OST48 mutants in which the C-terminal lysine-3 or lysine-5, but not lysine-7, had been replaced by leucine (OST48 DeltaK) could be detected on the cell surface. This indicates that these two lysine residues are sufficient for conferring ER-residency on OST48. The double-lysine motif operates only when exposed cytosolically, where it acts as a relocation signal rather than causing retention. OST48 DeltaK-3, when co-expressed in COS-1 cells together with myc-tagged ribophorin I, was quantitatively retained in the ER. By contrast, co-expression in the presence of ribophorin I resulted in no reduction of cell surface fluorescence for the OMO DeltaK-5 chimera containing the cytosolic and transmembrane domain of OST48 attached to the C-terminus of the Man(9)-mannosidase luminal domain. Thus ER-localisation of OST48 is probably brought about by complex formation with ribophorin I and this most likely involves the luminal domains of both proteins. Consequently, the double-lysine motif in the cytosolic domain of OST48 is unlikely to have a primary function except being involved in re-capture of molecules which have escaped from the ER.