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Lecture 6
O-GalNac GlycansC-type Lectins
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The Glycome is Rich
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Examples of O-Glycans
Ser/Thr
GalNAc
Mucins
Ser/Thr
Man
Yeast mannoproteins-dystroglycan
Ser/Thr
Fuc
NotchCoagulation FactorsFibrinolytic Factors
Ser/Thr
GlcNAc
Nuclear ProteinsCytoplasmic Proteins
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More O-glycans
Ser
Glc
Proteoglycans(Glycosaminglycans)
Ser
Xyl
Thr
Rho proteinsWorm CollagensDictyostelium proteins
Man Glc Gal GlcNAc
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Even more….
HydroxyLys
Gal
CollagenC1q complement
Plant glycoproteinsDictyostelium
HydroxyPro
Ara Gal GlcNAc
Tyr
Glycogenin
Glc
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O-Glycosidic Linkage
O-glycosidic linkage is sensitive to alkali (regardless of stereochemistry)
-elimination
O
OH
H
H
HO
H
O
NAcHH
OH
NH2CH
C
H2C
OSer
GalNAc
:OH
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Fritz, Timothy A. et al. (2004) Proc. Natl. Acad. Sci. USA 101, 15307-15312
Ribbon drawing of the ppGaNTase-T1 crystal structure
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Fritz, Timothy A. et al. (2004) Proc. Natl. Acad. Sci. USA 101, 15307-15312
Modeled binding of a MUC1 glycopeptide to ppGaNTase-T1
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Core 3
Ser/Thr
3
Core 4
Ser/Thr
3 6
Ser/Thr
3
Core 1
Ser/Thr
3 6
Core 2
Core 7
Ser/Thr
6
Core 6?
Ser/Thr
6
Core 5
Ser/Thr
3
Core 8
Ser/Thr
3
Other Core O-Glycan Structures
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Ser/Thr
3
T-antigen
Ser/Thr
3GalT
Tn-antigen
6GlcNAcT 3 6
Ser/Thr
Core 2
3
3
ST3Gal-I, II, IV
36
disialylT-antigen
ST6GalNAcIII, IV, I, II
sialyl Tn-
antigen6
ST6GalNAc-I
3 6sialyl
T-antigen
ST6GalNAc-II, I
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Cosmc
• Tn-antigens accumulate due to loss of 3Gal transferase activity
• No mutations in 3GalT; message is expressed
• Missing Cosmc (Core 1 3GalT -specific molecular chaperone, Xq23
• 25% identity, >40% homology to 3GalT
• Absence of Cosmc results in proteosome degradation of 3GalT
Cosmc
Ju & Cummings (2002) PNAS 99:16613-8
Ser/Thr
Tn-antigen
Ser/Thr
3
T-antigen
3GalT
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His-tagged Cosmc
HPC4 tagged T-synthase
Ni+ purification, Western with mAb to HPC4
T-syn
thas
eT-s
ynth
ase
+
Mut C
osmc
T-syn
thas
e +
Wt C
osmc
To
tal
No
t b
ou
nd
to
Ni+
Bo
un
d t
o N
i+
Is Cosmc a Chaperone?
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Outer Chain Assembly
• Sequential action of 4GalT and 3GlcNAcT gives rise to polylactosamine chains (Type II repeats)
• Type I repeats (Gal3GlcNAc3) also occur
• GlcNAc6Gal branches (I-antigen) can occur
• The ends of the chains are capped in -linked sugars, e.g. 3/4Fuc and 3/6sialic acids
• Terminal structures make up important blood group determinants, e.g. the Lewis antigens
4
3
4
3
4
3
3
3
Ser/Thr
3 6
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Intestinal Epithelium stained with Periodic acid-Schiff’s Reagent
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Mucins
• 11p15 family (MUC2, MUC5AC, MUC5B, MUC6) probably responsible for the formation of mucus layers
• 7q22 family (MUC3A, MUC3B, MUC12), 1q21 (MUC1), and 3q (MUC4, MUC13) are membrane mucins
• Mucin families are not necessairly related to each other, convergent evolution
Dekker et al. (2002) TIBS 27:126
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C-type Lectins
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Leukocyte Rolling
Tim Springer, Harvard
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