Oligosaccharyltransferase or OST () is a
membrane protein
Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
that transfers a 14-
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
oligosaccharide
An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a saccharide polymer containing a small number (typically two to ten) of monosaccharides (simple sugar ...
from
dolichol
Dolichol refers to any of a group of long-chain mostly unsaturated organic compounds that are made up of varying numbers of isoprene units terminating in an α-saturated isoprenoid group, containing an alcohol functional group.
Functions
Dolich ...
to nascent
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
. It is a type of
glycosyltransferase
Glycosyltransferases (GTFs, Gtfs) are enzymes ( EC 2.4) that establish natural glycosidic linkages. They catalyze the transfer of saccharide moieties from an activated nucleotide sugar (also known as the "glycosyl donor") to a nucleophilic glycos ...
. The sugar Glc
3Man
9GlcNAc
2 (where Glc=
Glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
, Man=
Mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylation ...
, and GlcNAc=
''N''-acetylglucosamine) is attached to an
asparagine
Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
(Asn) residue in the sequence Asn-X-
Ser
Ser or SER may refer to:
Places
* Ser, a village in Bogdand Commune, Satu Mare County, Romania
* Serpens (Ser), an astronomical constellation of the northern hemisphere
* Serres, known as Ser in Serbian, a city in Macedonia, Greece
Organization ...
or Asn-X-
Thr where X is any amino acid except
proline
Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the prot ...
. This sequence is called a glycosylation ''
sequon A sequon is a sequence of consecutive amino acids in a protein that can serve as the attachment site to a polysaccharide, frequently an N-linked-Glycan.
The polysaccharide is linked to the protein via the nitrogen atom in the side chain of asparagin ...
.'' The reaction catalyzed by OST is the central step in the
''N''-linked glycosylation pathway.
Location
![OST PM-1](https://upload.wikimedia.org/wikipedia/commons/b/bc/OST_PM-1.jpg)
OST is a component of the
translocon The translocon (also known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transport ...
in the
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
(ER)
membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...
. A lipid-linked core-oligosaccharide is assembled at the membrane of the endoplasmic reticulum and transferred to selected
asparagine
Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
residues of nascent
polypeptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A p ...
chains by the oligosaccharyl transferase
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
.
The active site of OST is located about 4 nm from the lumenal face of the ER membrane.
It usually acts during
translation
Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
as the nascent protein is entering the ER, but this cotranslational glycosylation is nevertheless called a
posttranslational modification
Post-translational modification (PTM) is the covalent and generally enzymatic modification of proteins following protein biosynthesis. This process occurs in the endoplasmic reticulum and the golgi apparatus. Proteins are synthesized by ribosome ...
. A few examples have been found of OST activity after translation is complete.
Current opinion is that post-translational activity may occur if the protein is poorly
folded or folds slowly.
Structure and function
Yeast OST is composed of eight different membrane-spanning proteins in three subcomplexes (one of them is
OST4
In molecular biology, OST4 (Dolichyl-diphosphooligosaccharide—protein glycosyltransferase subunit 4) is a subunit of the oligosaccharyltransferase complex.
OST4 is a very short, approximately 30 amino acids, protein found from fungi to vertebr ...
).
These octomers do not form higher order
oligomer
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
s, and three of the eight proteins are glycosylated themselves.
OST in mammals is known to have a similar composition.
OST is thought to require many subunits because it must:
#Position itself near the translocon pore.
#Recognize and bind oligosaccharyldolichol.
#Scan the nascent protein in order to recognize and bind sequons.
#Move these two large substrates into their proper locations and conformations.
#Activate the
Asn amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
atom for the actual transfer of oligosaccharide.
#Release its substrates.
The catalytically active subunit of the OST is called STT3. Two paralogs exist in eukaryotes, termed
STT3A and
STT3B. STT3A is primarily responsible for cotranslational glycosylation of the nascent polypeptide as it enters the lumen of the endoplasmic reticulum whereas STT3B can also mediate posttranslational glycosylation.
The structure of
PglB, the prokaryotic homolog of STT3 has been solved.
The high sequence similarity between the prokaryotic and the eukaryotic STT3 suggests that their structures are similar.
Clinical significance
CDG syndromes are
genetic disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
s of the glycosylation pathway. They are labelled "Type I" if the defective
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
is for an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
involved in the assembly or transfer of the Glc
3Man
9GlcNAc
2-dolichol precursor. They are labelled "Type II" if the defective step occurs after the action of OST in the
''N''-linked glycosylation pathway or involves
''O''-linked glycosylation.
See also
*
STT3A
*
STT3B
References
External links
*
{{Glycoprotein metabolism enzymes
EC 2.4.1