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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