15.10:
Tail-anchoring of Proteins in the ER Membrane
Tail-anchored or TA proteins are a special class of proteins anchored to the ER membrane via a single hydrophobic, transmembrane domain present at their C-terminal.
Their transmembrane domain also acts as an ER signal sequence that helps in the protein recruitment to the ER membrane.
As soon as a TA protein exits the ribosome, it's C terminal domain is recognized by Sgt2, a protein chaperone.
The Sgt2 then forms a pre-targeting complex with Get4 and Get5 to shield the hydrophobic domain of the target protein.
Next, the pre-targeting complex interacts with the ATP-bound Get3 ATPase via Get4.
Get3 protein acts as a chaperone and guides the protein to the Get1/Get2 heterotetrameric complex present on the ER membrane.
A cytosolic domain of the Get2 proteins then captures the Get3-TA protein complex.
This interaction is further stabilized by the Get1 cytosolic domain.
Hydrolysis of the ATP bound to the Get3 ATPase switches its conformation to an open state aiding the TA protein transfer to the Get1/Get2 complex.
Subsequently, the Get1/Get2 complex inserts the protein into the ER membrane, and the Get3 ATPase is recycled back to the cytosol.
The resultant tail-anchored protein has a free N-terminal in the cytosol, while the C-terminal is embedded into the ER membrane.
15.10:
Tail-anchoring of Proteins in the ER Membrane
Tail-anchored, or TA, proteins are estimated to make up to 3-5% of membrane proteins found in the eukaryotic cell. Such proteins have a single transmembrane domain located approximately 30 amino acid residues upstream from the C-terminal end. As a result, the signal recognition particle (SRP) cannot guide a TA protein to the ER membrane for cotranslational insertion. Hence, they are integrated into the ER membrane post-translationally using their C-terminal end as the anchor. TA proteins include membrane proteins such as Sec61ß and cytochrome b(5).
Canonical insertion pathway for insertion of TA proteins into the ER membrane
In yeast, the targeting and insertion of TA proteins into the ER occurs extensively using the GET (Guided Entry of Tail-anchored proteins) pathway. Get3 is a homodimeric ATPase that alternates between two ATP-dependent conformations. Get3 ATPase carries the TA protein cargo to the ER membrane. The association of Get3 with Get2 results in the formation of the heterotetrameric Get1/Get2 membrane complex that acts as a TA protein insertase. Cryo-electron microscopy (cryo-EM) and native mass spectrometry studies have improved the structural understanding of the arrangement of the Get proteins in the ER membrane. However, the detailed molecular characterization of the insertion of TA proteins into ER membrane still needs to be elucidated.
In contrast to yeast, a more complex homologous TRC40 (Transmembrane Recognition Complex of 40kDa) pathway is present in mammals.
Alternate membrane insertion pathways for TA proteins
While the Get pathway is involved in the membrane anchoring of proteins with moderately hydrophobic TA domains, other alternate pathways may come into play depending on the varying hydrophobicity of the TA domains. For example, highly hydrophobic TA domains are directed to the ER membrane post-translationally by an SRP-dependent pathway, which pairs with the Sec61 channel for lateral membrane insertion. On the other hand, proteins with low hydrophobicity of the TA domain opt for chaperone-mediated membrane insertion using HSP70/40 molecules. In addition, there is a more recently discovered SRP-independent (SND)-targeting pathway that comprises three leading players. In this pathway, SND1 is a cytoplasmic component, while SND2 and SND3 are membrane-bound proteins. SND2 and SND3 interact with Sec61 and Sec63 proteins to anchor the TA protein delivered onto the ER membrane by SND1. Thus, the canonical GET pathway is not the exclusive mode of TA protein insertion into the ER membrane as per the reports in yeast and mammalian cells.
Suggested Reading
- Hegde, Ramanujan S., and Robert J. Keenan. "Tail-anchored membrane protein insertion into the endoplasmic reticulum." Nature Reviews Molecular Cell Biology 12, no. 12 (2011): 787-798.
- Shan, Shu-ou. "Guiding tail-anchored membrane proteins to the endoplasmic reticulum in a chaperone cascade." Journal of Biological Chemistry 294, no. 45 (2019): 16577-16586.