Compartmentalization, Specialization, and Coordination: Protein Synthesis

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Compartmentalization, Specialization, and
CoordinationProtein Synthesis
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Protein Synthesisthere are generally 4 possible
destinationsfor proteins synthesized by the cell

• The cytoplasm structural proteins, non-digestive
  enzymes.
• Membrane-bound Organelles (Lysosomes) digestive
  enzymes.
• Phospholipid Bilayer transport proteins,
  antigens,
• Transport Vesicles for Export Out of the Cell
  hormones, digestive enzymes, neurotransmitters.

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How does the cell know?

• The Signal Hypothesis proteins have intrinsic
  signals that govern their transport and
  localization in the cell.

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Signal Hypothesis

• As a polypeptide is synthesized, amino acids at
  the NH2 end, or sometimes within the polypeptide,
  act as an address tag.
• Depending on the signal sequence, a polypeptide
  reaches a specific destination (organelle) within
  the cell.

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mRNA is synthesized in the nucleus and then
transported out to the cytoplasm
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In the cytoplasm, ribosomes attach to the mRNA
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The signal sequence, interacting with other
proteins in the cytoplasm will direct the protein
to the rough endoplasmic reticulum.
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In the lumen of the ER system, the polypeptide is
modified and transported to a terminal cysternae
(transitional ER) for packaging into a transport
vesicle.
transport vesicle
terminal cysternae/transitional ER
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The transport vesicle then travels to the
cis-face of the Golgi body and fuses with it.
Modification of the polypeptide continues as it
passes through the different compartments of the
Golgi body.
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The modified protein is packaged into a secretory
vesicle at the trans face of the Golgi apparatus
Secretory Vesicle
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If the protein is tagged for transport out of the
cell, it is dumped out of the cell by exocytosis.
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Summary
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The protein may also be tagged for packaging into
a lysosome
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or Incorporated into Cellular Membranes