The Transmembrane Domain of the Molecular Chaperone Cosmc Directs Its Localization to the Endoplasmic Reticulum

Abstract

The molecular basis for retention of integral membrane proteins in the endoplasmic reticulum (ER) is not well understood. We recently discovered a novel ER molecular chaperone termed Cosmc, which is essential for folding and normal activity of the Golgi enzyme T-synthase. Cosmc, a type II single-pass transmembrane protein, lacks any known ER retrieval/retention motifs. To explore specific ER localization determinants in Cosmc we generated a series of Cosmc mutants along with chimeras of Cosmc with a non-ER resident type II protein, the human transferrin receptor. Here we show that the 18 amino acid transmembrane domain (TMD) of Cosmc is essential for ER localization and confers ER retention to select chimeras. Moreover, mutations of a single Cys residue within the TMD of Cosmc prevent formation of disulfide-bonded dimers of Cosmc and eliminate ER retention. These studies reveal that Cosmc has a unique ER-retention motif within its TMD and provide new insights into the molecular mechanisms by which TMDs of resident ER proteins contribute to ER localization.

The MARVEL transmembrane motif of occludin mediates oligomerization and targeting to the basolateral surface in epithelia

“…This observation is an indication of the intricate relationship between oligomerization of proteins and their export from the ER. It has been reported that oligomerization of the Cosmc chaperone mediates its retention in the ER (Sun et al, 2011). Here, the BiFC-stable oligomers were retained in the ER while their mCherry counterparts were fully transport-competent.…”

Molecular Cloning of a Xylosyltransferase That Transfers the Second Xylose to O-Glucosylated Epidermal Growth Factor Repeats of Notch

“…COSMC is an ER-localized chaperone specific for the Golgi glycosyltransferase T-synthase and is itself a type II transmembrane protein with homology to the same T-synthase. It resides in the ER due to formation of a homodimer by a disulfide bond within the transmembrane domain that is essential for ER retention (44). Indeed, we observed dimer formation for XXYLT1 (Fig.…”

Identification of a Novel Protein Binding Motif within the T-synthase for the Molecular Chaperone Cosmc

“…Here we used a variety of molecular and biochemical approaches to define key aspects of the molecular basis for the specific recognition of the T-synthase by Cosmc and its role as a regulator of T-synthase folding. Our results demonstrate that Cosmc, an ER resident protein (46), directly binds to a unique hydrophobic region within the N-terminal stem region of T-synthase, which we have termed CBRT. Furthermore, in the absence of Cosmc in cells, the T-synthase cannot fold to become active, and thus any hydrophobic sequences within the T-synthase that might contribute to binding by other chaperones do not assist in correct folding of the enzyme in the absence of Cosmc.…”