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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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