Glycoproteins are
post-translationally modified proteins that take
part in nearly every biological process and make up a large percent
of the proteome. N-Linked glycosylation can be performed by
N
-glycosyltransferase (NGT), which recognizes the consensus
amino acid sequence, -Asn-X-Ser/Thr- (NXT), within the protein. The
enzyme catalyzes glycosidic bond formation between the oligosaccharide
donor, containing nucleoside phosphatase, and the amide nitrogen of
the asparagine residue. The attachment of the sugar moiety can influence
physiological and biological properties of the protein by affecting
their folding, modulating interactions with other biomolecules, and
modifying their functions at the cellular level. We are specifically
interested in the properties of membrane glycoproteins, which are
key components in a number of different disease states. Therefore,
the use of
in vitro
protein glycosylation can help
further evaluate the effects of the properties for these important
macromolecules.
In vitro
studies of N-linked glycosylation
were done in a stepwise fashion in a membrane-mimetic environment
to confirm that the methods for glycosylating soluble proteins could
be applicable to membrane proteins. Detergent and lipid systems were
used since hydrophobic peptides and membrane proteins are insoluble
in aqueous solvents. The stepwise method consisted of the glycosylation
of a soluble 7-residue peptide, a hydrophobic WALP-NVT peptide, and
a γ-sarcoglycan membrane protein, all of which contained the
glycosylation site Asn-Val-Thr (NVT). Glycosylation of the samples
was performed using
Escherichia coli
-expressed NGT from the
Actinobacillus pleuropneumoniae
genome, and a single sugar moiety of glucose, provided from a nucleotide-linked
donor, was added to the glycosylation site. Gel electrophoresis, mass
spectrometry, and NMR studies were used for the detection of glycosyltransferase
activity and to show the attachment of a single glucose molecule.
Our experiments demonstrated that small or large membrane proteins
that contain an N-glycosylation consensus sequence can be glycosylated
by NGT in membrane-mimetic environments.