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Protein Structure and Function
Electron micrograph of insect flight tissue
In cross section shows an array of 2 protein filaments
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DNA polymerase III – DNA complex (Replication)
Structure and Flexibility indicates Function
Conformational change of lactoferrin upon binding of Fe
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Cys can cross-link between 2 polypeptide chains -> Disulfide bridge
Covalent cross-link on 3° structure level
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Examples of α-Helical Proteins:
α-helical coiled coil proteins:
Form superhelix
Found in myosin, tropomyosin (muscle), fibrin (blood clots), keratin (hair)
The cytoskeleton is rich in filaments which are α-helical coiled coil proteins
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Examples of α-Helical Proteins:
Many membran proteins are α-helical
Bacteriorhodopsin (Photoreceptor)
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Examples of β-sheet Proteins:
Fatty acid binding protein -> β barrels structure
AntibodiesOmpX: E. coli porin
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Quaternary Structure:
Polypeptide chains assemble into multisubunit structures
Cell-surface receptor CD4
Cro protein phage λ
Tetramer of hemoglobin Coat protein of rhinovirus
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Protein Folding
Folding is a highly cooperative process (all or none)
Folding by stabilization of Intermediates
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Synthesis of secretory proteins and their cotranslational translocation across the ER membrane
What is needed for translocation:
1.Signal sequence (9-12 hydrophobic AA with some mainly pos. charged ones – in some prokaryotes sometimes longer, most of the times cleaved off by peptidases on the ER lumen side, sequence mainly at N-terminal)
2.Signal-Recognition-Particle (SRP) –recognizes signal sequence of ribosome complex (ribosome with mRNA), redirects ribosome complex to SRP receptor, puts synthesis of protein on hold
3.SRP receptor – binds the ribosome- SRP complex - driggers that ribosome complex is moved to translocon (GTP dependent)
4.Translocon is a protein channel, opens upon binding of ribosome complex, synthesis through channel
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GPI-anchored Proteins
Glycosylphosphatidylinositol (GPI) From yeastIn other organisms -> differs in1.Acyl chain2.Carbohydrate moiety
Formation of GPI-anchored proteins in the ER membrane
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Protein Modification
Membrane and soluble secretary proteins synthesized on the ER have 4 possible modifications before the reach final destination:
1. Glycosylation in ER and Golgi
2. Formation of S-S bonds in ER
3. Proper folding and assembly of multisubunits in ER
4. Proteolytic cleavage in ER, Golgi, and secretory vesicles
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Protein Modification - Glycosylation
O-linked glycosylkation:
Attachment of sugars to OH of Ser and Thr
Often contain only 1-4 sugar groups
N-linked glycosylation:
Attachment of sugars to amine N of Asn (Asn-X-Ser/Thr)
Larger and more sugar groups -> more complex
Glycosylation patters differ slightly between spieces !!!
In Yeast:
N-linked glycosylation are classified as core and mannan types. The core type contains 13-14 mannoses whereas the mannan-type structure consists of an inner core extended with an outer chain of up to 200-300 mannoses, a process known as hyperglycosylation.
Precursor of N-linked sugars that are added to proteins in the ER
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Processing of N-linked glycoproteins in the Golgi apparatus of mammalien cells
Mannose trimming
Gucosamine addition
Galactose addition + neuraminic acid linkage to galactose
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Folding and assembly of Multimers
Hemagglutinin trimer folding
Binding of Chaperone BiP
Closing S-S bond, N-linked glycosylation
Membrane anchoring
Assembly of trimer
Another example for assembly of multimers -> immunoglobulins
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Improperly Folded Protein Induce Expression of Chaperons
Unfolded and incomplete folded protein in the ER-> releases chaperons (BiP) from Ire1-> upon release of BiP Ire1 dimerizes (activation) -> Endonuclease activity in th cytosol -> splices Transcription factor Hac1 -> Hac1 protein returns into nucleus -> activates transcription of Chaperons
-> Misfolded and unassembled proteins -> transported from the ER to the cytosol -> degradation
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Modification of Proteins - Proteolytic Cleavage
Proteolytic cleavage of proinsulin occurs in secretory vesicles (after Golgi)
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Export of Bacterial Proteins
Post-translational translocation across inner membrane of gram-negative bacteria
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Injection of Protein by Pathogenic Bacteria (into Animal cells)
Secretion mechanism for injecting bacterial proteins into Eukaryotic cells
Yersinia pestis:
Causes Pest
Virulence: Disables host macrophages
-> by injecting a small set of proteins into macrophages
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Protein Transport between Organelles are done by Vesicles
Assembly of protein coat drives vesicle formation and selection of cargo molecules
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Assembly and Disassembly of Coat protein
Interaction of cargo protein with vesicleN-terminus of Sar1 (membrane anchor) not shown
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Vesicle-mediated Protein Trafficking between ER and Golgi
Backtransport mainly used for:
-> recycling of membrane bilayer-> recycling of proteins (SNARE)-> missorted proteins
Normal transport of secretory proteins
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Membrane Fusion directed by Hemagglutinin (HA)
Influenza Virus:
Glycoprotein on suface of virus
After endocytosis (uptake of virus of the cell) viral envelop fuses with endosomal membrane
Acidic pH necessary for conformational change in HA -> viral HA can insert into endosomal
membrane
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