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.
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