Protein Production and Sorting

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  • Created by: rosieevie
  • Created on: 23-05-17 15:11

Protein Expression and Targeting

Organelle-specific uptake targeting sequences - part of primary structure that acts as address label, allowing access into target organelles for sorting

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The Secretory Pathway

Translocation occurs at ER membrane

Vesicle transport links ER with cis-golgi

Maturation occurs through medial golgi - linked by vesicles

Sorting of proteins occurs at trans-golgi domain

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Translocation at ER

Secretory pathway begins with protein synthesis on ER membrane

  • Protein produced by mRNA has ER signal sequence
  • SRP recongises and binds
  • SRP also binds SRP receptor on ER membrane
  • Ribosome binds
  • SRP transfers ribosome/protein to translocon membrane protein
  • SRP recycled
  • Protein translocated to ER lumen as translation occurs
  • Signal sequence cleaved off protein
  • Ribsosome dissociates
  • Protein modifications occur within ER
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Membrane Protein Insertion

Membrane proteins occur in different orientations/transversing numbers

More times a protein transverses = more stable (embedded)

Stop-transfer anchor sequence contains hydrophobic amino acids that interact w/ phospholipids and allow insertion into membrane

  • Internal signal sequence recognised by SRP

Positioning of signal sequence in primary structure affects orientation of membrane proteins

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Modifying ER Proteins

Newly synthesised ER proteins modified 

Folding with disulfide bonds, glycosylation, proteolytic cleavage, multimer formation

Protein transported to golgi by vesicular delivery

KDEL receptors recognise KDEL sequences of proteins that should be functioning in ER

= Transported back to ER and concentrated

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Lysosomal Protein Targeting

Lysosome proteins glycosylated and manose-6 residue phosphorylated

Residue identified by M6P receptor on trans-golgi domain 

Receptor changes conformation

Clatharin binds to cytoplasmic domain = mesh-like pit around receptor

= Vesicle coated in clatharin 

Clatharin subunits dissociate and vesicles form early lysosome w/ low pH

Low pH caused by association between manose-6P and protein to be lost

M6P receptor recycled back to golgi

Transport vesicle formed around protein

= LYSOSOME

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Epithelial Cell Protein Targeting

Golgi packages proteins designed for apical membrane into separate vesicles than those travelling to basolateral membrane

Achieved by receptor surface interactions, similar to lysosomal protein targeting

Vesicles attach to microtubules on cytoskeleton and use ATP to transport to required membrane

If proteins attached to wrong membrane, binding pathways aren't present and protein digested or endocytosed

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Protein Glycosylation

Occur in both rough ER and cis/medial domains of golgi

Always begins in ER

Carbohydrate attachment forms wide diversity of proteins

Dolichol lipids deliver oligosaccharide trees onto asparagine residues as translation occurs

Enzymes in ER/golgi add or remove carbohydrates to contribute to tree's specific composition

In golgi, protein groups e.g. UDP or CMP carry monosaccharides to tree for addition

Unique glycosylation patterns criticial to protein function

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LDL Endocytosis

LDL (lipid and cholesterol) binds to cluster of LDL receptors

Forms clathrin occupied pit = vesicle

Clathrin dissociated = acidic pH forms 

Leads to receptor and LDL particle dissociating and receptor recycled

LDL particle delivered to lysosome - degraded using enzymes

Products released

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