Asparagine-linked glycosylation is the most ubiquitous protein
co-translational modification in the endoplasmic reticulum (ER).1
The enzyme that catalyzes this process is called oligosaccharyl
transferase (OT). It catalyzes the transfer of an oligosaccharyl
moiety (Glc3Man9GlcNAc2) from the dolichol-linked pyrophosphate
donor to the side chain of Asn within a consensus sequence of
Asn-X-Thr/Ser, where X can be any amino acid residue except for
Pro (1GÇô3). This modification serves as a primary determinant for
specific molecular recognition as well as protein folding and stability
(4, 5) and therefore is an essential and highly conserved protein
modification pathway in eukaryotic cells. It has been established in
both yeast and higher eukaryotic organisms that this enzyme exists
as a heteromeric, multisubunit complex in the ER membrane.
In the last decade, because of the facility of yeast genetics, genes
encoding 9 OT subunits in Saccharomyces cerevisiae have been
cloned and identified (for review see Ref. 2 and earlier studies cited
therein). Among them, the five genes encoding Ost1p, Ost2p, Stt3p,
Wbp1p, and Swplp are essential for the viability of the cell; the OST4
gene is essential for growth of the cell at 37 -¦C but not at 25 -¦C;
Ost3p, Ost5p, and Ost6p subunits are not essential for the viability of
the yeast cell but are required for maximal enzyme activity.
At present, many mammalian OT subunit proteins have also been
identified, and some of them share high sequence identity and similarity
with yeast homologs. Four of them were isolated from highly
purified and active enzyme fractions: ribophorin I (homolog of yeast
Ost1p), ribophorin II (homolog of yeast Swp1p), OST48 (homolog of
yeast Wbp1p), and DAD1 (homolog of yeast Ost2p) (6GÇô10). Recently
STT3-A and STT3-B (homologs of yeast Stt3p), N33 and implantation-
associated protein (homologs of yeast Ost3p and Ost6p, respectively),
as well as OST4 (homolog of yeast Ost4p) have been characterized
by genome-wide searches (11). These proteins have been
demonstrated to be assembled together with ribophorin I, ribophorin
II, OST48, and DAD1 into a multimeric complex similar to the yeast
OT (11).
Although genetic studies have yielded considerable information on
this enzyme complex, one of the fundamental questions, the enzymatic
mechanism of N-glycosylation, has remained unanswered.
Specifically, the substrate recognition and/or catalytic sites, the role
of each of the subunits, and how they interact structurally has continued
to be obscure. In this review, we focus on explorations developed
within the past 5 years to clarify the mechanism of this highly
conserved protein modification pathway as well as the function of
each of the subunits.

Asparagine-linked glycosylation is the most ubiquitous protein
co-translational modification in the endoplasmic reticulum (ER).1
The enzyme that catalyzes this process is called oligosaccharyl
transferase (OT). It catalyzes the transfer of an oligosaccharyl
moiety (Glc3Man9GlcNAc2) from the dolichol-linked pyrophosphate
donor to the side chain of Asn within a consensus sequence of
Asn-X-Thr/Ser, where X can be any amino acid residue except for
Pro (1GÇô3). This modification serves as a primary determinant for
specific molecular recognition as well as protein folding and stability
(4, 5) and therefore is an essential and highly conserved protein
modification pathway in eukaryotic cells. It has been established in
both yeast and higher eukaryotic organisms that this enzyme exists
as a heteromeric, multisubunit complex in the ER membrane.
In the last decade, because of the facility of yeast genetics, genes
encoding 9 OT subunits in Saccharomyces cerevisiae have been
cloned and identified (for review see Ref. 2 and earlier studies cited
therein). Among them, the five genes encoding Ost1p, Ost2p, Stt3p,
Wbp1p, and Swplp are essential for the viability of the cell; the OST4
gene is essential for growth of the cell at 37 -¦C but not at 25 -¦C;
Ost3p, Ost5p, and Ost6p subunits are not essential for the viability of
the yeast cell but are required for maximal enzyme activity.
At present, many mammalian OT subunit proteins have also been
identified, and some of them share high sequence identity and similarity
with yeast homologs. Four of them were isolated from highly
purified and active enzyme fractions: ribophorin I (homolog of yeast
Ost1p), ribophorin II (homolog of yeast Swp1p), OST48 (homolog of
yeast Wbp1p), and DAD1 (homolog of yeast Ost2p) (6GÇô10). Recently
STT3-A and STT3-B (homologs of yeast Stt3p), N33 and implantation-
associated protein (homologs of yeast Ost3p and Ost6p, respectively),
as well as OST4 (homolog of yeast Ost4p) have been characterized
by genome-wide searches (11). These proteins have been
demonstrated to be assembled together with ribophorin I, ribophorin
II, OST48, and DAD1 into a multimeric complex similar to the yeast
OT (11).
Although genetic studies have yielded considerable information on
this enzyme complex, one of the fundamental questions, the enzymatic
mechanism of N-glycosylation, has remained unanswered.
Specifically, the substrate recognition and/or catalytic sites, the role
of each of the subunits, and how they interact structurally has continued
to be obscure. In this review, we focus on explorations developed
within the past 5 years to clarify the mechanism of this highly
conserved protein modification pathway as well as the function of
each of the subunits.

MEAT MAST

HAM CANNON

SAUSAGE FLAGPOLE

ALL WORDS FOR MY ****

MEAT MAST

HAM CANNON

SAUSAGE FLAGPOLE

ALL WORDS FOR MY ****

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OH GOD I'M RUNNING OUT OF STEAM

OH GOD I'M RUNNING OUT OF STEAM

POSTING IS HARD AND MY **** NO LONGER IS