Three Mechanisms of Protein Import into Organelles

• Gated Transport

  • Protein traffic between nucleus and cytosol

  • Nuclear pore acts as a gate

• Transmembrane Transport

  • Membrane bound protein translocators directly transport proteins across a membrane

  • Cytosol 🡪 ER, mitochondria, chloroplast

  • Transported protein is usually unfolded

• Vesicular Transport

  • Membrane-enclosed transport intermediates

    • Vary in size

    • Ferry proteins from one compartment to another

Large vs Small Proteins

• Transport of large molecules is active - requires GTP

  • Binding to transporter

  • Transport through nuclear pore using GTP

• Small molecules (< 60 kDa, or about 9 nm diameter) enter or exit nucleus by passive diffusion

Protein Import Into Mitochondria

1. Chaperone proteins keep protein unfolded.

2. Matrix-targeting sequence binds to receptor.

3. Chaperones are released as protein is transferred to a channel in the outer membrane

4. Protein is transferred to a channel in the inner membrane.

5. Chaperones bind to protein as it enters the matrix

6. Matrix-targeting sequence is cleaved by an enzyme in the matrix.

7. Protein is completely threaded into the matrix.

8. Chaperones are released, and protein folds into its three-dimensional structure.

Protein Transport Into the ER Lumen

1. SRP binds to ER signal sequence and pauses translation.

2. SRP binds to receptor in ER membrane.

3. SRP is released, and translation resumes. The growing polypeptide is threaded into a channel.

4. The ER signal sequence is cleaved by signal peptidase.

5. The polypeptide is completely synthesized and released into the ER lumen.