Protein%20targeting%20or%20Protein%20sorting%20Refer%20Page%201068%20to%201074%20%20Principles%20of%20Biochemistry%20by%20Lehninger%20

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Protein targeting or Protein sortingRefer
Page 1068 to 1074 Principles of Biochemistry by
Lehninger Page 663 Baltimore Mol Cell Biology
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• Protein targeting or Protein sorting is the
  process of delivery of newly synthesized
  proteins to their proper cellular destinations
• Proteins are sorted to the endoplasmic reticulum
  (ER), mitochondria, chloroplasts, lysosomes,
  peroxisomes, and the nucleus by different
  mechanisms
• The process can occur either during protein
  synthesis or soon after synthesis of proteins by
  translation at the ribosome.
• Most of the integral membrane proteins, secretory
  proteins and lysosomal proteins are sorted to ER
  lumen from where these proteins are modified for
  further sorting

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• For membrane proteins, targeting leads to
  insertion of the protein into the lipid bilayer
• For secretory/water-soluble proteins, targeting
  leads to translocation of the entire protein
  across the membrane into the aqueous interior of
  the organelle.
• Protein destined for cytosol simply remain where
  they are synthesized
• Mitochondrial and chloroplast proteins are first
  completely synthesized and released from
  ribosomes. These are then bound by cytosolic
  chaperone proteins and delivered to receptor on
  target organelle.
• Nuclear proteins such as DNA and RNA polymerases,
  histones, topoisomerases and proteins that
  regulate gene expression contain Nuclear
  localization signal (NLS) which is not removed
  after the protein is translocated. Unlike ER
  localization signal sequence which is at N
  terminal, NLS ca be located almost anywhere along
  the primary sequence. Most NLS consist of four to
  eight amino acid residues with consecutive basic
  (Arg or Lys) residues

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• Proteins sorted to ER contains amino terminal
  Signal sequence which translocates these proteins
  to lumen of ER
• The function of Signal Sequence was first
  proposed by G Blobel in 1970
• The signal sequence can be 13 to 36 amino acids
  residues
• 10 to 15 residues are hydrophobic amino acids
• There is one or more positively charged amino
  acid near amino terminal preceding hydrophobic
  residues
• A short polar sequence at the carboxyl terminus
  near cleavage site (eg Ala residue)
• George Palade demonstrated that proteins with ER
  signal sequence are synthesized on ribosomes
  attached to ER (rough ER) and Signal sequence
  helps to direct ribosomes to ER

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Directing eukaryotic proteins with the
appropriate signals to the Endoplasmic Reticulum
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Directing eukaryotic proteins with the
appropriate signals to the Endoplasmic Reticulum
Directing eukaryotic proteins with the
appropriate signals to the Endoplasmic Reticulum

• The protein targeting pathway begins with
  initiation of protein synthesis on free
  ribosomes. The ER signal sequence appears early
  in protein synthesis because it is at the amino
  terminus.
• Once the ER signal sequence emerges from the
  ribosome, it is bound by a signal-recognition
  particle (SRP)
• The SRP delivers the ribosome/nascent polypeptide
  complex to the SRP receptor in the ER membrane.
  This interaction is strengthened by binding of
  GTP to both the SRP and its receptor
• Transfer of the ribosome/nascent polypeptide to
  the translocon (peptide translocation complex)
  leads to opening of this translocation channel
  and insertion of the signal sequence and adjacent
  segment of the growing polypeptide into the
  central pore

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• Both the SRP and SRP receptor, once dissociated
  from the translocon, hydrolyze their bound GTP
  and then are ready to initiate the insertion of
  another polypeptide chain
• As the polypeptide chain elongates, it passes
  through the translocon channel into the ER lumen,
  where the signal sequence is cleaved by signal
  peptidase and is rapidly degraded
• Once translation is complete, the ribosome is
  released, the remainder of the protein is drawn
  into the ER lumen, the translocon closes, and the
  protein assumes its native folded conformation

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Glycosylation Plays a Key Role in Protein
Targeting

• Following the removal of signal sequences,
  polypeptides are folded, disulfide bonds formed,
  and many proteins glycosylated to form
  glycoproteins
• In many glycoproteins the linkage to their
  oligosaccharides is through Asn residues.
• These N-linked oligosaccharides are diverse, but
  the pathways by which they form have a common
  first step.
• A 14 residue core oligosaccharide is built up in
  a stepwise fashion, then transferred from a
  dolichol phosphate donor molecule to certain Asn
  residues in the protein
• The transferase is on the lumenal face of the ER
  and thus cannot catalyze glycosylation of
  cytosolic proteins
• After transfer, the core oligosaccharide is
  trimmed. All N-linked oligosaccharides retain a
  pentasaccharide core derived from the original 14
  residue oligosaccharide.

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Dolichol phosphate

• A few proteins are O-glycosylated in the ER, but
  most O-glycosylation occurs in the Golgi complex
  or in the cytosol
• Several antibiotics act by interfering with one
  or more steps in this process and have aided in
  elucidating the steps of protein glycosylation.
  The best-characterized is tunicamycin, which
  mimics the structure of UDP-N-acetylglucosamine
  and blocks the first step of the process

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• The presence of one or more mannose 6-phosphate
  residues in its N-linked oligosaccharide is the
  structural signal that targets the protein to
  lysosomes
• A receptor protein in the membrane of the Golgi
  complex recognizes the mannose 6-phosphate signal
  and binds the hydrolase so marked.
• Vesicles containing these receptor-hydrolase
  complexes bud from the trans side of the Golgi
  complex and make their way to sorting vesicles.
• Here, the receptor-hydrolase complex dissociates
  in a process facilitated by the lower pH in the
  vesicle and by phosphatase-catalyzed removal of
  phosphate groups from the mannose 6-phosphate
  residues.
• The receptor is then recycled to the Golgi
  complex, and vesicles containing the hydrolases
  bud from the sorting vesicles and move to the
  lysosomes.

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Phosphorylation of mannose residues on
lysosome-targeted enzymes. N-Acetylglucosamine pho
sphotransferase recognizes some as yet
unidentified structural feature of hydrolases
destined for lysosomes.
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Bacteria Also Use Signal Sequencesfor Protein
Targeting

• Bacteria can target proteins to their inner or
  outer membranes, to the periplasmic space between
  these membranes, or to the extracellular medium.
  They use signal sequences at the amino terminus
  of the proteins
• Most proteins exported from E. coli make use of
  the following pathway

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Bacteria Also Use Signal Sequencesfor Protein
Targeting
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• Model for protein export in bacteria.
• 1 A newly translated polypeptide binds to the
  cytosolic chaperone protein SecB, which
• 2 delivers it to SecA, a protein associated with
  the translocation complex (SecYEG) in the
  bacterial cell membrane.
• 3 SecB is released, and SecA inserts itself into
  the membrane, forcing about 20 amino acid
  residues of the protein to be exported through
  the translocation complex.
• 4 Hydrolysis of an ATP by SecA provides the
  energy for a conformational change that causes
  SecA to withdraw from the membrane, releasing the
  polypeptide.
• 5 SecA binds another ATP, and the next stretch of
  20 amino acid residues is pushed across the
  membrane through the translocation complex.
• Steps 4 and 5 are repeated until 6 the entire
  protein has passed through and is released to the
  periplasm.
• The electrochemical potential across the membrane
  (denoted
• by and - ) also provides some of the driving
  force required for protein translocation.