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Protein targeting and sorting
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Overview of sorting of nuclear-encoded proteins in
eukaryotic cells
Two basic forms of targeting pathways
Eukaryotic targeting signals
to endoplasmic reticulum ( via to all secretory pathway)
to mitochondria
to chloroplast
to peroxiosme
Common principles in protein translocation across
membranes
Protein targeting and sortingHeadlines
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Protein sortingProtein sortingOverview of sorting of nuclearOverview of sorting of nuclear--encoded proteins in eukaryotic cellsencoded proteins in eukaryotic cells
Molecular Cell Biology. Lodish et als. Figure 17-1
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Question
How the cytosolic made proteins are
guided towards the correct locationsince multiplex proteins are targeted to
different organelles?
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Protein traffic
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Protein Trafficking/TargetingProtein Trafficking/Targeting
Protein targetingisnecessary for proteinsthat are destined to workoutside the cytoplasm.
Protein targetingis morecomplex in eukaryotes
because of the presenceof many intracellularcompartments.
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EuEukaryotickaryotic protein targetingprotein targeting
In the absence of targeting signals, a protein will remain in the cytoplasm
translational machinery
metabolic enzymes
cytoskeletal proteins
many signal transduction proteins
The signals involved are also
called sortingsignals. They are
regions on the targeted proteinwith certain amino acid
sequences
These signals interact withspecific receptors, either on the
target organelle or a carrier
protein
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Two basic forms of targeting pathways
post-translationaltargeting:
nucleusmitochondria
chloroplasts
Peroxisomes
co-translationaltargeting (secretory
pathway):
ER
Golgilysosomes
plasma membrane
secreted proteins
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Targeting signals
(Fig12-8 Alberts)
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Eukaryotic targeting signals
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Mitochondria
(Prof. Ruth Bellairs, Department of Anatomy and Developmental Biology,
University College, London WC1E 6BT)
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Eukaryotic targeting signals
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Mitochondrial targeting signals
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Eukaryotic targeting signals
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Chloroplast targeting signals
1st Structure1st Structure :
- contain hydroxylated aa.
- small hydrophobic aa.
- positively charged aa.
- few acidic aa.
22ndnd StructureStructure:
- random coiled insolution
- -sheet
The ability to perform conformational
changes might be an important feature for
cytosolic targeting
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Chloroplast targeting signals.Chloroplast targeting signals.
Sequence logosSequence logos
Cleavage site for peptidase
Sequence of mature proteinSequence from targeting signal
(O. Emanuelsson et als. 1999 Protein Sci 1999. 8(5):978-984)
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Transport of peroxisomal proteinsHow to get a folded protein across the membrane?
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Eukaryotic targeting signals
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The PTS1 motif
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The PTS1 motif
Peroxisomal/Non-peroxisomal occurrence
HL 3 / 5 PHL 1 /8
KA 1 /19 PKL 3 / 4KF 1 / 3 PRL 1 / 3
KI 1 / 9 SHL 10 / 18KL 19 / 21 SKL 2 / 7KM 3 / 1 SKI 6 / 2KV 1 /19 SKL 42 / 20NL 2 / 5 SKM 4 / 1RF 1 / 2 SKV 1 / 24RL 4 / 9 SNL 2 / 9RM 5 / - SQL 4 / 4RY 1 / 1 SRL 14 / 11
SL 1 / 23 SRM 4 / -CKL 2 / 4 THL 1 / 18HRL 1 / 1 TKL 1 / 23HRM 2 / 1 TKV 1 / 4
KKL 2 / 6 YRM 1 / 13NKL 4 / 10
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Peroxisomal membrane protein import
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Types of targeting signal sequences
Target Organelle Usua l SignalLocation within
Protein
SignalRemoval*
Nature of Signal
Endoplasmicreticulum
N-terminal (+) Core of 612 mos tly hydrophobic am inoacids, often precede d by one or mo re basicamino acids
Mitochondrion N-terminal (+) 3 5 nonc onsecu tive Arg or Lys residues,often w ith Ser and T hr; no Glu or As p residues
Chloroplast N-terminal (+) No com mo n sequenc e motifs; gene rally rich inSer, Thr, and sm all hydrop hobic amino ac idresidues and poor in G lu and Asp residues
Peroxisome C-terminal (?) Usu ally Ser-Lys-Leu at extreme C-terminus
Nucleus Internal (?) On e cluster of 5 basic amino acids, or twosma ller clusters of basic residues separated by? 10 amino acids
Molecular Cell Biology. Lodish et als. Table17-1
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Translocation across membranesCommon principles1. Multiple pathways of protein
translocation across membrane
exit.
2. Molecular chaperones are required
in the cytosol, inside the organelle
and often within the organelle
membrane.
3. ATP and/or GTP hydrolysis is often
required.
4. A proton-motive force across themembrane is often required.
5. Protein translocation occurs
through gated, aqueous channels