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.
