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Cell Biology Lecture 223.17: Intracellular Compartments and

Protein Sorting

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Protein Trafficking

Protein Trafficking - Signals

“signal peptide”

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Protein Trafficking - Signal Hypothesis

Protein Trafficking - Gated

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• Selective cage

• Small molecules diffuse freely • Macromolecules actively transported

Protein Trafficking - Gated

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Targeting Signals in the Protein (1) Signal Peptide

• 15-60 Aa • Anywhere in sequence • Bind to receptor on organelle • Cleaved by signal peptidase

-Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val- (Table 15-3)

E.g. nuclear localization signal (4-8 aa)

Protein Trafficking - Signals

Protein Trafficking - Gated

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Protein Trafficking - Gated

Protein Trafficking - Gated

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Protein Trafficking

Protein Trafficking -Transmembrane

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Transmembrane Transport • E.g.1 Cytosol to ER

• Smooth ER: -no ribosomes -lipid synthesis -detoxification

• Rough ER: -studded with ribosomes -involved in synthesis of proteins carrying an ER import signal* -specifically, synthesis of proteins for endomembrane system (other ER and organelles) and cell surface

Protein Trafficking -Transmembrane

Targeting Signals in the Protein (1) Signal Peptide

• 15-60 AAs • Bind to receptor on organelle • Cleaved by signal peptidase

+H3N-Met-Met-Ser-Phe-Val-Ser-Leu-Leu-Leu-Val-Gly-Ile-Leu-Phe-Trp-Ala-Thr-Glu-Ala-Glu-Gln-Leu-Thr-Lys-Cys-Glu-Val-Phe-Gln-

(Table 15-3)

E.g. Import to ER (N-terminus)

Protein Trafficking - Signals

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Protein Trafficking -Transmembrane

Two types of ribosomes in cytosol: 1.  Membrane-bound ribosomes (attached to ER) – next slide 2.  Free ribosomes (unattached to any membrane) –see above Polyribosome = a complex of mRNA + 2 or more ribosomes (can be free or bound)

Proteins with no ER signal remain in cytosol *

Protein Trafficking -Transmembrane

At the end of each round, the ribosome subunits rejoin the common pool

When a ribosome makes a protein with a ER signal sequence (red), the signal sequence directs the ribosome to the ER membrane

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Protein Trafficking -Transmembrane

Two components guide the ER signal sequence: 1. Signal recognition particle (SRP) and 2. SRP receptor

Protein Trafficking -Transmembrane

(or protein translocator)

Note: bound ribosome is present but not shown

Soluble protein has signal sequence

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Protein Trafficking -Transmembrane: Single Pass Single-pass transmembrane protein with start and stop transfer sequences

Single pass-Glycophorin

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Protein Trafficking -Transmembrane: Double Pass

Fig 15-17 A double-pass transmembrane protein has an internal ER signal sequence

Protein Trafficking -Transmembrane: Multi Pass

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Transmembrane Transport •  Once in the ER, proteins can be further shuttled to other locations

• But what if it is supposed to stay in the ER?

• Signal sequence for ER retention

-Lys-Asp-Glu-Leu-COO-

Protein Trafficking -Transmembrane

Protein Trafficking -Transmembrane: Peroxisomes

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Protein Trafficking -Transmembrane: Peroxisomes

Import signal sequence -Ser-Lys-Leu-COO-

Import of proteins from cytosol is mediated by a 3 aa signal at the C-terminus of the protein (see above)

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Protein Trafficking