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Department of Molecular Biology & Pharmacology Washington University School of Medicine St. Louis, Missouri 63110
The placental hormone human CG (hCG) consists of
two noncovalently linked 
- and ß-subunits similar to the other
glycoprotein hormones LH, FSH, and TSH. These heterodimers share a
common subunit but differ in their structurally distinct ß
subunits. The CGß subunit is distinguished among the ß subunits by
the presence of a C-terminal extension with four serine-linked
oligosaccharides (carboxyl terminal peptide or CTP). In previous
studies we observed that deleting this sequence decreased assembly of
the truncated CGß subunit (CGß114) with the -subunit and
increased the heterogeneity of the secreted forms of the uncombined
subunit synthesized in transfected Chinese hamster ovary (CHO) cells.
The latter result was attributed to alterations in the processing of
the two N-linked oligosaccharides. To examine at what step this
heterogeneity occurs, the CGß and CGß114 genes were transfected
into wild-type and mutant CHO cell lines that are defective in the late
steps of the N-linked carbohydrate-processing pathway. We show here
that removal of the CTP alters the processing of the core mannosyl unit
of the subunit to complex forms at both glycosylation sites and that
the oligosaccharides contain polylactosamine. Although it has been
presumed that there is little intramolecular interaction between the
CTP and the proximal domains of the subunit, our data suggest that the
CTP sequence participates in the folding of the newly synthesized
subunit, which is manifest by the posttranslational changes observed
here.
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