1. Post-Translational Modification

2. Disparity in mRNA and Protein profiles Electrophoresis 18(1997)533-537
Splicing variants
In eukaryotic cells, likely 6-8 proteins/gene
Post-translational modification
22 different forms of antitrypsin observed in human plasma

3. Posttranslational Modification
What is it ?
Addition of groups or deletion of parts to make a finished protein
What groups ? How much ? Where ?
- methyl
- acetyl
- glyco
- phospho

4. Posttranslational Modification
What purpose ?
- targeting (eg. some lipoproteins)
- stability (eg. secreted glycoproteins )
- function (eg. surface glycoproteins)
- control of activity (eg. clotting factors, caspases)
How can we study it ?

5. Post-Translational Modifications

6. Post-Translational Modifications

7. Posttranslational Modification Modification Charge-dependent change
Acylation loss of a-amino positive charge
Alkylation alteration of a- or e-amino positive group
Carboxylmethylation esterification of specific carboxyl group
Phoshorylation mainly modify Ser, Thr and Tyr
Sulfation mainly modify Tyr
Carboxylation bring negative charge
Sialyation mainly on Asn, Thr and Ser
Proteolytic processing truncation leads to change of pI

8. Posttranslational Modification Location Modification
Nucleus acetylation, phosphorylation
Lysosome mannose-6-phosphate labelled N-linked sugar
Mitochondria N-formyl acylation
Golgi N- and O-linked ologosaccharide, sulfation,
palimitoylation
ER N-linked oligosaccharide, GPI-anchor
Cytosol acetylation, methylation, phosphorylation,
Ribosome myristoylation
Plasma membrane N- and O-glycosylation, GPI-anchor
Extraceullar fluid N- and O-glycosylation, acetylation,
phosphorylation
Extrallular matrix N- and O-glycosylation, phosphorylation,
hydroxylation

9. Posttranslational Modification
Examples:
Chromatin Structure/function - acetylation
Regulation of mitochondrial processes –
phosphorylation
Evade immune system – glycosylation
Gene regulation – glycosylation
Recognition - glycosylation

10. Protein Glycosylation
The most important and complex form of PTM
Approx. 1% mammalian genes
Early view about carbohydrates (non-specific, static structures) has been challenged
Ann. Rev. Biochem. 72(2003)643

11. Protein Glycosylation
Which proteins are decorated with glycans
(polysaccharides) ?
What are the structures of these glycans?
What is their functional significance?

12. Protein Glycosylation Common in Eukaryotic Proteins

13. Protein Glycosylation - Biological Significance
Oligosaccharides may be a tissue-specific marker
Carbohydrates may alter the polarity and solubility
Steric interaction between protein and oligosaccharides dictates certain protein 3D structure
The bulkiness and negative charge of oligosaccharide chain may protect protein from the attack by proteolytic enzymes

14. The Sugar Code Carbohydrates as Informational Molecule
Information: intracellular targeting of proteins, cell-cell interactions, tissue development, extracellular signals
Improved methods for structural analysis
Sugar code - The unique complex structure
of oligosaccharide on glycoprotein read by protein

15. Lectins carbohydrate-binding proteins
Lectins read sugar code and mediate many biological processes :
[1] Cell-cell recognition
[2] Signaling
[3] Adhesion
[4] Intracellular targeting of newly synthesized
proteins