18238212 Posttransl. ExploreCommunity ational Modifications to Proteins

8/14/2019 18238212 Posttransl. ExploreCommunity ational Modifications to Proteins

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Post-translational

Modifications to Proteins

Affecting Form and Function


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Contents

Why are proteins modified?GlycosylationMembrane proteinsProteolytic processingPhosphorylationAcetylationSmall Molecule BindingRegulated degradation


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Why are proteins modified?

Regulation of activity modification may turn activity on modification may turn activity off modification may generate a different function

Protein-protein interaction modification site may be a binding interface

Subcellular localization modification site may be a targeting signal modification may be a membrane anchor

Aging modification may identify the protein for degradation modification may target a protein to be scavenged


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Topics to cover in my section

593-595 Membrane proteins - myristlyation, farnesylation

598,612-3,735-736 Glycosylation

702-704 Glycosylation in rough ER

706,763-764 GPI anchors

733-735 Oligosaccharide processing (Golgi & ER, Glyco-related)

742-745 M6P in lysosomal enzymes

1085-6 Lectins and the immune system 1091-6 Proteoglycans and extra-cellular matrix

678-681,685,694 Cleavage of signal sequences (mitochondria, chloroplasts, ER)

760 Proteolysis of secreted proteins (to activate them)

893-895 Notch cleavage in neuron development


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Glycosylation

Major form of protein modification

Sugars are added in the ER and Golgi

Most proteins formed in the ER areglycoproteins

Many different forms and functions


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Initial glycosylation in the ER

A precursoroligosaccharide

is formed on adolichol lipid

This is

transferred tothe growing

protein


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Glycosylation and protein folding

by glucosidase


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Processing in the Golgi


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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


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Functions of glycosylation

Provide sorting signalsM6P for lysosomal hydrolases

GPI anchors (see later)

Contributes to differentiation events in

organism developmentRemoving N-acetylglucoasmine transferase I in

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

asymmetry impaired


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Proteoglycans and the Extracellular

MatrixMade of core protein and

polysaccharide chains

Extremely diverseForm hydrated gel

Resists compressive forces

Regulate traffic (perlecan in kidney)

Can regulate secretedprotein activity e.g. chemokines in inflammatory

response


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Membrane proteins: GPI anchors

GPI-anchored proteins are delivered tothe apical plasma membrane

Trypanosomes can shed these proteins

to avoid immune attack


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Myristylation and Farnesylation

Attachescytosolic

proteins tothe plasmamembrane

Proteinusually

involved in

signal

transduction


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Proteolytic processing

Why is this common for secretedenzymes?

Some peptides (e.g. enkephalins) too shortby themselves

Prevent premature activation of hydrolytic

enzymes


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Phosphorylation

Most commonposttranslational modificationto proteins in eukaryotes

Enzymes and regulators are

turned on and off Energy from ATP


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Phosphorylation Regulates Protein

Synthesis eIF-2


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Phosphorylation and Molecular

Switches


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Signalling using GTP-Binding - Ras

Protein

Broadcasts signals from cell surfaceCell proliferation

Differentiation


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Phosphorylation and Motor Proteins

Move chromosomes during

mitosis

Move organelles along

molecular tracks

Move enzymes along DNA

during DNA synthesis


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Phosphorylation and Motor Proteins

ATP binding - conformation 1to conformation 2

ATP hydrolyzed to ADP Pi-

conformation 2 toconformation 3.

Release of ADP and Pi-

back to conformation 1.Irreversible one direction

only


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Acetylation and Histones

Acetylation enhances

transcription

Deacetylation

represses transcription


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Small Molecule Binding

A. Retinal

B. Heme group


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Protein Degradation

Degradationmechanisms:Ubiquitin ligase

Degradation signalMultiubiquitin

chain marksprotein fordegradation inproteosome


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Common Post-translational Modifications

Deoxyhexoses

Hexoses

Sialic acid

Pentoses

Hexosamines

N-acetylhexosamines

Carbohydrates

SulphationPhosphorylationHydroxyl groups

DeamidationPyroglutamic acid

Carboxylation

Acids & amides

Formylation

Lipoic acid

MyristoylationPalmitoylation

Geranylgeranylation

Methylation

Acetylation

FarnesylationBiotinylation

Stearoylation

Amines

Oxidation

Glutathionylation

Disulphide bond

Cysteinylation

Sulphydryls


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18238212 Posttransl. ExploreCommunity ational Modifications to Proteins

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