Post-translational Modifications to Proteins

Affecting Form and Function

Contents

Why are proteins modified? GlycosylationMembrane proteinsProteolytic processingPhosphorylation AcetylationSmall Molecule BindingRegulated degradation

Glycosylation

Major form of protein modificationSugars are added in the ER and GolgiMost proteins formed in the ER are

glycoproteinsMany different forms and functions

Initial glycosylation in the ER

A precursor oligosaccharide is formed on a dolichol lipid

This is transferred to the growing protein

Glycosylation and protein folding

by glucosidase

Processing in the Golgi

Functions of glycosylation

Stabilise proteins against proteolysis Limit approach of macromolecules to protein

surface

Modulation of immune response Selectins (weakly) bind to oligosaccharides Helps to concentrate lymphocytes in lymphoid

organs Attracts white blood cells & platelets to

inflammation sites

Functions of glycosylation

Provide sorting signals M6P for lysosomal hydrolases GPI anchors (see later)

Contributes to differentiation events in organism development Removing N-acetylglucoasmine transferase I in

mice causes embryo deathNeural tube development and left-right body plan

asymmetry impaired

Proteoglycans and the Extracellular MatrixMade of core protein and

polysaccharide chains Extremely diverse

Form hydrated gel Resists compressive forces Regulate traffic (perlecan in kidney)

Can regulate secreted protein activity

e.g. chemokines in inflammatory response

Membrane proteins: GPI anchors

GPI-anchored proteins are delivered to the apical plasma membrane

Trypanosomes can shed these proteins to avoid immune attack

Myristylation and Farnesylation

Attaches cytosolic proteins to the plasma membrane

Protein usually involved in signal transduction

Proteolytic processing

Why is this common for secreted enzymes? Some peptides (e.g. enkephalins) too short by

themselves Prevent premature activation of hydrolytic

enzymes

Phosphorylation

Most common posttranslational modification to proteins in eukaryotes

Enzymes and regulators are turned ‘on’ and ‘off’

Energy from ATP

Phosphorylation Regulates Protein Synthesis – eIF-2

Phosphorylation and Molecular Switches

Signalling using GTP-Binding - Ras ProteinBroadcasts signals from cell surface

Cell proliferation Differentiation

Phosphorylation and Motor Proteins

Move chromosomes during mitosis

Move organelles along molecular tracks

Move enzymes along DNA during DNA synthesis

Phosphorylation and Motor Proteins

ATP binding - conformation 1 to conformation 2

ATP hydrolyzed to ADP Pi - conformation 2 to conformation 3.

Release of ADP and Pi - back to conformation 1.

Irreversible – one direction only

Acetylation

Acetylation and Histones

Acetylation enhances transcription

Deacetylation represses transcription

Small Molecule Binding

A. Retinal

B. Heme group

Protein Degradation

Degradation mechanisms: Ubiquitin ligase Degradation signal

Multiubiquitin chain marks protein for degradation in proteosome

Common Post-translational Modifications

Sulphydryls Disulphide bondCysteinylation

OxidationGlutathionylation

Amines Methylation Acetylation Farnesylation Biotinylation Stearoylation

Formylation Lipoic acid Myristoylation Palmitoylation Geranylgeranylation

Acids & amides Pyroglutamic acid Carboxylation

Deamidation

Hydroxyl groups Phosphorylation Sulphation

Carbohydrates Pentoses Hexosamines N-acetylhexosamines

Deoxyhexoses Hexoses Sialic acid

Summary

Post-translational modifications – necessary for protein function Correct protein folding Organism development Cellular Signalling Motor Proteins Regulating degradation …and much more…