title: Lysine can be replaced by histidine but not by arginine as the ER retrieval motif for type I membrane proteins.
abstract: The OST48 subunit of the oligosaccharyltransferase complex is a type I membrane protein containing three lysines in its cytosolic domain. The two lysines in positions 3 and 5 from the C-terminus are able to direct protein localisation within the endoplasmic reticulum (ER) by COPI-mediated retrieval. Substitution of these lysines by arginine resulted in cell-surface expression of OST48, whereas ER residency was maintained when either Lys-5 or Lys-3 but not both was replaced with arginine. Localisation of OST48 was not affected by substitution of the two lysines by histidine, indicating that a His-Xaa-His sequence, in contrast to Arg-Xaa-Arg, contains ER-specific targeting information. These differences show that simple charge interactions are not sufficient for ER retention and that other structural factors also play a role. The His-Xaa-His sequence could represent a new and independent signal for directing ER localisation differing from both the arginine motif in type II proteins and the lysine motif in type I proteins. Our data do not exclude, however, that the histidine sequence simply mimics the lysine motif as a sorting signal, being recognised by and interacting with the same receptor subunit(s) in COP-I-coated vesicles. Conclusions arising from this assumption involving the conformation of lysine at the putative COP-I binding site and the failure of Arg-Xaa-Arg to mediate ER localisation for type I proteins are discussed.
authors.affiliation: Institut für Physiologische Chemie, Universität Bonn, Nussallee 11, 53115 Bonn, Germany.
authors.forename: Birgit
authors.initials: B
authors.lastname: Hardt
authors.forename: Ernst
authors.initials: E
authors.lastname: Bause
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D001120:Arginine / Q000737:chemistry / Q000235:genetics; D019556:COS Cells; D004721:Endoplasmic Reticulum / Q000737:chemistry*; D006602:Hexosyltransferases; D006639:Histidine / Q000737:chemistry* / Q000235:genetics; D008239:Lysine / Q000737:chemistry* / Q000235:genetics; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008856:Microscopy, Fluorescence; D017434:Protein Structure, Tertiary; D014166:Transferases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D008565:Membrane Proteins; D006639:Histidine; D001120:Arginine; D014166:Transferases; D006602:Hexosyltransferases; C025620:dolichyl-diphosphooligosaccharide - protein glycotransferase; D008239:Lysine