Protein Transport
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
- Chaperone proteins keep protein unfolded.
- Matrix-targeting sequence binds to receptor.
- Chaperones are released as protein is transferred to a channel in the outer membrane
- Protein is transferred to a channel in the inner membrane.
- Chaperones bind to protein as it enters the matrix
- Matrix-targeting sequence is cleaved by an enzyme in the matrix.
- Protein is completely threaded into the matrix.
- Chaperones are released, and protein folds into its three-dimensional structure.
Protein Transport Into the ER Lumen
- SRP binds to ER signal sequence and pauses translation.
- SRP binds to receptor in ER membrane.
- SRP is released, and translation resumes. The growing polypeptide is threaded into a channel.
- The ER signal sequence is cleaved by signal peptidase.
- The polypeptide is completely synthesized and released into the ER lumen.