THE TRANSMEMBRANE DOMAIN OF A CARBOXYL-TERMINAL ANCHORED PROTEIN DETERMINES LOCALIZATION TO THE ENDOPLASMIC-RETICULUM

UBC6 is a C-terminal membrane-anchored (type IV) protein, native to Sa ccharomyces cerevisiae, where it is found in the endoplasmic reticulum . When expressed in mammalian cells, this novel ubiquitin-conjugating enzyme also localizes to the endoplasmic reticulum. UBC6 lacks a lumen al domain and contains no known endoplasmic reticulum retention signal s. Analysis of chimeric proteins in which the cytosolic domain of UBC is linked to a heterologous transmembrane domain, or in which the UBC6 transmembrane domain is appended to an unrelated soluble protein, led to the determination that the transmembrane domain of UBC6 plays a do minant role in its compartmental localization. The basis for the trans membrane domain-mediated subcellular targeting of UBC6 was evaluated b y lengthening the wild type UBC6 hydrophobic segment from 17 to 21 ami no acids, which resulted in re-targeting to the Golgi complex. A furth er increase in length to 26 amino acids allowed this modified protein to traverse the secretory pathway and gain expression at the plasma me mbrane. These findings are consistent with models in which, in the abs ence of dominant cytosolic or lumenal targeting determinants, proteins may be sorted within the secretory pathway based on interactions betw een their trans membrane domains and the surrounding lipid bilayer.