The signal sequence interacts with the SRP

KEY TERMS:

• Protein translocation describes the movement of a protein across a membrane. This occurs across the membranes of organelles in eukaryotes, or across the plasma membrane in bacteria. Each membrane across which proteins are translocated has a channel specialized for the purpose.
• The signal recognition particle (SRP) is a ribonucleoprotein complex that recognizes signal sequences during translation and guides the ribosome to the translocation channel. SRPs from different organisms may have different compositions, but all contain related proteins and RNAs.
• Signal peptidase is an enzyme within the membrane of the ER that specifically removes the signal sequences from proteins as they are translocated. Analogous activities are present in bacteria, archaebacteria, and in each organelle in a eukaryotic cell into which proteins are targeted and translocated by means of removable targeting sequences. Signal peptidase is one component of a larger protein complex.

KEY CONCEPTS:

• The signal sequence binds to the SRP (signal recognition particle).
• Signal-SRP binding causes protein synthesis to pause.
• Protein synthesis resumes when the SRP binds to the SRP receptor in the membrane.
• The signal sequence is cleaved from the translocating protein by the signal peptidase located on the "inside" face of the membrane.

Protein translocation can be divided into two general stages: first ribosomes carrying nascent polypeptides associate with the membranes; and then the nascent chain is transferred to the channel and translocates through it.

The attachment of ribosomes to membranes requires the signal recognition particle (SRP). The SRP has two important abilities:

• It can bind to the signal sequence of a nascent secretory protein.
• And it can bind to a protein (the SRP receptor) located in the membrane.

  The SRP and SRP receptor function catalytically to transfer a ribosome carrying a nascent protein to the membrane. The first step is the recognition of the signal sequence by the SRP. Then the SRP binds to the SRP receptor and the ribosome binds to the membrane. The stages of translation of membrane proteins are summarized in Figure 8.21.

The role of the SRP receptor in protein translocation is transient. When the SRP binds to the signal sequence, it arrests translation (Walter and Blobel, 1981). This usually happens when ~70 amino acids have been incorporated into the polypeptide chain (at this point the 25 residue leader has become exposed, with the next ~40 amino acids still buried in the ribosome).

Then when the SRP binds to the SRP receptor, the SRP releases the signal sequence. The ribosome becomes bound by another component of the membrane. At this point, translation can resume. When the ribosome has been passed on to the membrane, the role of SRP and SRP receptor has been played. They now recycle, and are free to sponsor the association of another nascent polypeptide with the membrane (for review see Walter and Johnson, 1994).

This process may be needed to control the conformation of the protein. If the nascent protein were released into the cytoplasm, it could take up a conformation in which it might be unable to traverse the membrane. The ability of the SRP to inhibit translation while the ribosome is being handed over to the membrane is therefore important in preventing the protein from being released into the aqueous environment.

The signal peptide is cleaved from a translocating protein by a complex of 5 proteins called the signal peptidase. The complex is several times more abundant than the SRP and SRP receptor. Its amount is equivalent roughly to the amount of bound ribosomes, suggesting that it functions in a structural capacity. It is located on the lumenal face of the ER membrane, which implies that the entire signal sequence must cross the membrane before the cleavage event occurs. Homologous signal peptidases can be recognized in eubacteria, archaea, and eukaryotes (Tjalsma et al., 1998).