PROTEIN TARGETING

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Protein Targeting

• M.Prasad Naidu
• MSc Medical Biochemistry,
• Ph.D.Research Scholar

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• Each eukaryotic cell is subdivided into
  functionally distinct, membrane-bound
  compartments organelles
• Each compartment has its own distinct set of
  proteins functions
• A complex distribution system moves proteins from
  the place of synthesis to its proper destination

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Protein targeting

• Protein has to be correctly localized to perform
  proper function.
• Receptors plasma membrane
• DNA polymerase nucleus
• Catalase peroxisomes
• Insulin outside

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• All proteins begin to be synthesized on cytosolic
  ribosomes.
• Sorting or translocation can occur
• Co-translational
• Post-translational
• If the protein is for cytosolic functins, the
  synthesis will be finished on free ribosomes and
  the peptide is released into the cytosol.

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• If the protein is destined for nucleus,
• mitochondria or peroxisomes the synthesis is
• also finished on cytoplasmic ribosomes and the
  peptide is released to the cytosol (to be sorted
  later or post-translationally).
• If the protein is going to be secreted from the
  cell or it destined for the membranes the
  ribosome with the nascent peptide is targeted to
  the ER (ER becomes rough) and sorting is done
  during translation (co-translationally).

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• post-translational targeting
• nucleus
• mitochondria
• Peroxisomes
• co-translational targeting
• (secretory pathway)
• ER
• Golgi
• lysosomes
• plasma membrane
• secreted proteins

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Targeting sequence

• Characteristic for the destination not the
  protein
• Part of the polypeptide
• Can be cleaved later by signal peptidase or
  remain permanent part of protein
• Can be located on N-, C-terminus or in the middle
  of the protein

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BLOBEL SABATINI PROPOSED THE SIGNAL
HYPOTHESIS.( GUENTER BLOBEL NP 1999 )
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Transport of the new protein intomitochondria

• Most mitochondrial proteins are encoded by
  nuclear DNA
• Only very few are encoded by mitochondrial DNA
  and synthesized on mitochondrial ribosomes

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Mitochondrial targeting signals

• Usually located at N-terminus of precursor
  polypeptide
• Usually removed in mitochondrial matrix

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Receptor/translocation channels inmitochondria

• Tom translocase of the outer mitochondrial
  membrane
• Tim - translocase of the inner mitochondrial
  membrane

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• Mitochondrial proteins are synthesized in cytosol
  as precursors
• Bind to cytosolic chaperones(Hsp 70) to keep them
  unfolded until they ready to be translocated
• Energy from ATP

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• Some outer membrane proteins insert themselves in
  the membrane while in transit
• Intermembrane space proteins remain there and
  fold
• Protein destined to matrix passes through Tom 40
  and then Tim (inner membrane translocon)

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Peroxisomes

• Single membrane organelle
• Matrix contains oxidative enzymes
• Lipid oxidation without ATP production
• Proteins encoded by nuclear DNA (all have to be
  imported).

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Transport into peroxisomes

• Proteins are synthesized and fully folded in
  cytosol
• Fully functional, fully folded protein is
  transported!
• Import requires ATP hydrolysis
• Peroxisome targeting sequences
• PTS1 on C-terminus, very conserved
• PTS2 on N-terminus, just few proteins

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• Peroxins - peroxisome transport receptors
• Bind to proteins with PTS1 and dock to the
• translocation channel
• The complex is transported through the
• membrane
• Protein is released
• Peroxin is recycled

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Transport into the nucleus

• All proteins found in the nucleus are synthesized
  in the cytoplasm
• Examples
• Histones
• Ribosomal proteins
• DNA and RNA polymerases
• Transcription factors

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• Transport requires nuclear localization sequences
  (NLS)
• Transport occurs through the nuclear pores
• Nuclear import receptor (Importin a and ß)
• Energy from GTP
• GTPase Ran
• Fully folded proteins are transported

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• Importin a and ß bind to the protein to be
  transported
• Nuclear localization signal binds to importin a
• The complex is translocated through the nuclear
• membrane

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• Activated Ran (GTP) causes the complex to
  dissociate
• Ran transports importin ß back to cytosol
• Importin a becomes a part of export receptor

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Synthesis of secretory proteins and their
co-translationaltranslocation across the ER
membrane

• ER signal sequence emerges
• The binding by a signal-recongition particles
  (SRP)
• SRP delivers the ribosome/nascent polypeptide
• complex to the SRP receptor in the ER membrane,
  and GTP binding

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Major topological classes of integral membrane
proteins synthesized on the rough ER
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Diseases due to defective protein targeting

1. ZELLWEGER SYNDROME
2. PRIMARY HYPEROXALURIA
3. FAMILIAL HYPERCHOLESTROLEMIA
4. CYSTIC FIBROSIS
5. INCLUSION CELL DISEASE

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Thank you