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Two different mannose-binding proteins (MBP-A and MBP-C), which show 56%
sequence identity, are present in rat serum and liver. It has previously been
shown that MBP-A binds to a range of monosaccharide-bovine serum albumin
conjugates, and that, among oligosaccharide ligands tested, preferential binding
is to terminal nonreducing N-acetylglucosamine residues of complex type N-linked
oligosaccharides. In order to compare the binding specificity of MBP-C, an
expression system has been developed for production of a fragment of this
protein which contains the COOH-terminal carbohydrate-recognition domain. After
radioiodination, the domain has been used to probe natural glycoproteins,
neoglycoproteins, and neoglycolipids. Like MBP-A, MBP-C binds several different
monosaccharides conjugated to bovine serum albumin, including mannose, fucose,
and N-acetylglucosamine, although binding to the last of these is relatively
weaker than observed for MBP-A. The results of binding to natural glycoproteins
and to neoglycolipids containing oligosaccharides derived from these proteins
are most compatible with the interpretation that MBP-C interacts primarily with
the trimannosyl core of complex N-linked oligosaccharides, with additional
ligands being terminal fucose and perhaps also peripheral mannose residues of
high mannose type oligosaccharides. This binding specificity is thus quite
distinct from that of MBP-A. The presence of multiple MBPs with distinct binding
specificities in preparations derived from serum and liver explains conflicting
conclusions which have been reached about carbohydrate recognition by these
proteins.
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