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Overview of protein sorting

Introduction

Protein sorting involves targeting proteins within or outside the cell to specific intracellular locations such as organelles. The majority of proteins found in organelles are synthesised by ribosomes in the cytosol. These proteins require specific targeting signals in their amino acid sequence to transport them to the correct subcellular location.

Cytosol

Proteins which are destined for the cytosol do not require a targeting signal since they are already present in the cytosol. Therefore the cytosol can be considered as a 'default' for when a targeting signal is missing from a protein.

Nucleus

For a protein to be targeted to the nucleus it requires a nuclear localisation signal (NLS). This signal:

  • Can be found anywhere on the protein.
  • Is short in length.
  • Contains a large amount of basic, positively charged amino acids such as lysine and arginine.
  • Is not removed after the protein has been transported to the nucleus.

 

    The process of importing a protein into the nucleus requires a nuclear import receptor and the small GTPase, Ran. The import of a protein into the nucleus involves:

    • The binding of this protein to a nuclear import receptor in the cytosol.
    • This complex is then transported into the nucleus via the nuclear pore.
    • Ran with GTP bound (Ran-GTP) binds to the nuclear import receptor, releasing the protein.
    • The now empty receptor is transported back to the cytosol where the GTP is hydrolysed to GDP and inorganic phosphate (Pi) to form Ran-GDP.
    • Ran-GDP dissociates from the receptor.

     

      The diagram below illustrates this process.

      Protein import into the nucleus



      Mitochondria

      Proteins which are destined for the mitochondria are synthesised in the cytosol as the mitochondrial precursor proteins and have a mitochondrial targeting signal that is:

      • Found at the N-terminus of the protein.
      • Usually cleaved after import into the mitochondria by the mitochondrial processing peptidase.

       

        The signal also has a strong tendency to form an amphipathic alpha helix.

        Proteins are imported into the mitochondria, unfolded and post-translationally modified. They bind to receptors found in the outer mitochondrial membrane and are then threaded through the translocation channel into the mitochondria.

        Endoplasmic reticulum

        Endoplasmic reticulum (ER) targeting signals:

        • Are found at the N-terminus of the protein.
        • Are 8 or more amino acids in length.
        • Contain hydrophobic and non-polar amino acids.
        • Are cleaved after the protein has been transported into the ER by a signal peptidase.

         

          Membrane proteins targeted to the ER also have a stop-transfer sequence or a signal anchor sequence as well as a targeting signal. A stop-transfer sequence prevents the complete tranfer of a protein and so is anchored in the membrane. As with the targeting signal, the stop-transfer signal is cleaved by a signal peptidase. The signal anchor sequence also anchors the protein in the membrane but is not removed.

          The import of the protein into the ER, unlike other organelles, is co-translational. That is, the ribosome which synthesises the protein is directly attached to the ER membrane (rough endoplasmic reticulum). This allows the protein to be translocated into the ER while it is still being synthesised. However proteins can also be imported post translationally which involves:

          • A signal recognition particle (SRP) binding to the ER targeting signal on the protein.
          • This complex then binds to a SRP receptor in the ER membrane, releasing the SRP.
          • The targeting signal then binds to an ER translocation channel called SEC61 complex.
          • SEC61 complex opens and the targeted protein is threaded through into the ER.

           

            The ER, along with the Golgi apparatus, lysosome and plasma membrane, make up the secretory pathway. Again, signals are needed to transport the proteins from the ER to the correct location in the pathway, the table below shows this.

            Protein targeting post ER



            Peroxisome

            Although the import process is not yet fully understood, it is known that there are two signals which can target a protein to the peroxisome:

            • The most common signal is the SKL signal, made up of Ser-Lys-Leu and is found at the C-terminus.
            • The other signal is found at the N-terminus.

             

            Choroplast/Thylakoid

            Two signal sequences are required in order to target a protein to the thylakoid :

            • A chloroplast signal sequence at the N-terminus with a large amount of small, hydrophobic amino acids.
            • A second hydrophobic signal.

             

            Sequence of steps required to import a protein into the thylakoid:

            • Chloroplast signal sequence binds a receptor in the chloroplast outer membrane.
            • The protein is then translocated through a translocation channel into the stroma with the use of ATP or GTP.
            • Once inside the stroma, the chloroplast signal sequence is cleaved by stromal processing peptidase.
            • The thylakoid signal sequence is then exposed and transports the protein into the thylakoid.

            Diseases

            Cystic fibrosis is an example of an 'ER storage disease,' caused by a protein not being transported to its correct place. Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) result in its inability to pump chloride ions out of the cell, causing the symptoms of the disease. However in some cases the mutated CFTR is still functional but is retained in the ER instead of being transported to the plama membrane. It is the removal of the functional CFTR which causes cyctic fibrosis.

            References

            Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter. Molecular Biology of the Cell 5th Ed, pgs 701-783, Garland Science 2008.

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