Protein Destinations

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•  Free ribosomes synthesise proteins for use primarily within the cell

•  Bound ribosomes synthesise proteins primarily for secretion or for use in lysosomes 

All proteins produced by eukaryotic cells are initially synthesised by ribosomes found freely circulating within the cytosol

  • If the protein is targeted for intracellular use within the cytosol, the ribosome remains free and unattached
  • If the protein is targeted for secretion, membrane fixation or use in lysosomes, the ribosome becomes bound to the ER

Protein destination is determined by the presence or absence of an initial signal sequence on a nascent polypeptide chain

  • The presence of this signal sequence results in the recruitment of a signal recognition particle (SRP), which halts translation
  • The SRP-ribosome complex then docks at a receptor located on the ER membrane (forming rough ER)
  • Translation is re-initiated and the polypeptide chain continues to grow via a transport channel into the lumen of the ER
    • The synthesised protein will then be transported via a vesicle to the Golgi complex (for secretion) or the lysosome
    • Proteins targeted for membrane fixation (e.g. integral proteins) get embedded into the ER membrane
  • The signal sequence is cleaved and the SRP recycled once the polypeptide is completely synthesised within the ER

The Role of the Signal Recognition Particle in Protein Destination