Intracellular Traffic& Sorting of Proteins

Ashikh Seethy

Junior Resident

Dept of Biochemistry

MAMC – New Delhi

Ribosomes:

Overview:

• How proteins are targeted to their correct destinations?

• Clinical conditions associated with defects in protein targeting

• Drugs

• Mechanisms of certain toxins

• Maintenance of quality control in protein traffic

• Vesicle transport

• Degradation of proteins in proteasomes

A major sorting decision is made early during protein synthesis

Secretory pathway-mechanism

Gunter Blobel

• There is no difference between structure of free and bound ribosomes

• Selection of mRNA to the ER membrane is not via direct binding of the mRNA itself, but rather via binding of its nascent translation product

• Signal hypothesis

Signal peptide enables the binding of ‘bound ribosome’-The Signal Hypothesis

• N-terminal

• 13-36 residues

• 6-15 hydrophobic core flanked by hydrophilicresidues

• 1 or 2 basic residues near the N-terminal

• Small and neutral residues near the cleavagesite

Signal hypothesis:

Transmembrane proteins are of different classes:

Signal

sequence

Type I Transmembrane Protein

Type II, III & IV Transmembrane Proteins

GPI linked proteins

Some proteins are transported post-translationally

Quality Control in Endoplasmic Reticulum-ERAD

Clinical Significance

Unfolded Protein Response and DM

From ER to Golgi and Further

Vesicle transport

Coat proteins

Golgi>>>PM/Lysosomes ER>>>Golgi Golgi>>>ER

Fusion• R or v-SNARE

Synaptobrevin

• Q or t-SNARE Syntaxin

SNAP-25 [SynaptosomeAssociated Protein]

• Disassembly:

SNARE

SNAP

NSF

Fusion

Vesicle transport

ER resident proteins have a KDEL sequence

• C-terminal: KDEL

KKXX

KXKXXX

• "If found, please return to ER"

Targeting to lysosomes

I-cell disease

• Mucolipidosis II

• UDP-N -acetyl glucosamine phosphotransferase

• Cultured fibroblasts-deficient in numerous lysosomal enzymes

• Inclusions in lysosome

• These enzymes were found to be present in excess in tissue culture media and in extracellular fluids

• Psychomotor and skeletal defects

Cytosolic pathway

Protein import to peroxisomes:

• Peroxisomal matrix targeting sequences

• PTS-1:

SKL

• PTS-2:

N-terminal (R/K)(L/V/I)X5(H/Q)(L/A)

The Zellweger spectrum:

• Zellweger cerebrohepatorenal syndrome

• Neonatal adrenoleukodystrophy

• Infantile Refsum disease

• Peroxisomal biogenesis disorders

• Mutation in PEX genes Impaired plasmalogen synthesis

Impaired very long chain fatty acid (VLCFA) beta oxidation

Impaired alpha oxidation

Signal sequences for nuclear import are not cleaved

• Nuclear Localisation Signal– Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val

Protein targeting to mitochondria

Protein degradation

• 76 residues

• Highly conserved

• Isopeptide bonds

Juvenile onset Parkinsonism

HPV

• 26 S proteasome

20S core subunit

19S regulatory subunit

• Bortezomib and Carfilzomib

Summary

Thank

You.