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*
Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, CA 92037; and
Department of Immunology, Novartis Research Institute, Vienna, Austria
The high affinity IgE receptor (Fc
RI) is a multisubunit complex
comprised of either 
2 or 
2
chains. The cotranslational assembly of the IgE-binding -chain with
a dimer of -chains occurs in a highly controlled manner and is
proposed to involve masking of a dilysine motif present at the
cytoplasmic C terminus of the FcRI -chain that targets
localization of this subunit to the endoplasmic reticulum (ER). Here,
we show that ER quality control modulates export from the ER of newly
synthesized 2 and 2 receptors.
We demonstrate that the presence of untrimmed N-linked
core glycans (Glc3Man9GlcNAc2) on
the FcRI -chain activates the ER quality control mechanism to
retain this subunit in the ER, despite the presence of -chains. At
the same time, the untrimmed, ER-localized -chain exhibits
IgE-binding activity, suggesting that FcRI -chain folding occurs
before constitutive glucose trimming. In additional experiments, we
demonstrate that cell surface expression of an -chain C-terminal
truncation mutant is also dependent on glucose trimming, but not on
-chain coexpression. We suggest that glucosidase trimming of
terminal glucose residues is a critical control step in the export of
FcRI from the ER. Finally, we show that the constitutive ER
FcRI -chain, expressed in the absence of the other FcRI
subunits, associates with the ER lectin-like chaperone calnexin, but
not the structurally similar ER chaperone calreticulin, presumably
through interaction with monoglucosylated -chain ER
glycoforms.
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