Question 1

What are tail-anchored proteins and where are they located?

Accepted Answer

Tail-anchored (TA) proteins are a specialized class of membrane proteins with a single hydrophobic transmembrane domain at their C-terminal end, representing 3-5% of eukaryotic membrane proteins. Unlike typical membrane proteins, their transmembrane domain is positioned approximately 30 amino acids upstream from the C-terminus, allowing it to serve as both an anchor and an ER signal sequence. Examples include Sec61β and cytochrome b5.

Question 2

Why can't the signal recognition particle direct tail-anchored proteins to the ER?

Accepted Answer

The signal recognition particle (SRP) cannot guide TA proteins to the ER because their transmembrane domain is located at the C-terminal end rather than near the N-terminus. Since TA proteins are synthesized completely before their signal sequence emerges from the ribosome, cotranslational insertion is impossible. Instead, TA proteins require post-translational translocation of proteins to the ER using specialized pathways like the GET system.

Question 3

How does the GET pathway deliver tail-anchored proteins to the ER membrane?

Accepted Answer

The GET pathway begins when Sgt2, a protein chaperone, recognizes the C-terminal hydrophobic domain of a newly synthesized TA protein. Sgt2 forms a pre-targeting complex with Get4 and Get5 to shield the hydrophobic domain. This complex then interacts with ATP-bound Get3 ATPase, which carries the TA protein to the Get1/Get2 heterotetrameric complex on the ER membrane. ATP hydrolysis by Get3 triggers conformational change, enabling TA protein insertion into the membrane.

Question 4

What role does Get3 ATPase play in tail-anchored protein insertion?

Accepted Answer

Get3 is a homodimeric ATPase that alternates between two ATP-dependent conformations to transport TA protein cargo to the ER membrane. In its ATP-bound state, Get3 accepts the TA protein from the pre-targeting complex and guides it toward the Get1/Get2 insertase. When ATP is hydrolyzed, Get3 switches to an open conformation, facilitating TA protein transfer to the membrane complex. After insertion, Get3 is recycled back to the cytosol.

Question 5

What are the alternative pathways for tail-anchored protein insertion besides the GET pathway?

Accepted Answer

TA proteins with highly hydrophobic domains use an SRP-dependent pathway paired with the Sec61 channel for lateral membrane insertion. Proteins with low-hydrophobicity TA domains employ chaperone-mediated membrane insertion using HSP70/40 molecules. The SND-targeting pathway, comprising SND1, SND2, and SND3 proteins, represents a more recently discovered alternative. These pathways demonstrate that the canonical GET pathway is not the exclusive mode of TA protein insertion in yeast and mammalian cells.

Question 6

What is the final orientation of a tail-anchored protein after insertion into the ER membrane?

Accepted Answer

After successful insertion via the GET pathway, a tail-anchored protein has a free N-terminal domain extending into the cytosol while its C-terminal transmembrane domain is embedded within the ER membrane. This orientation is achieved because the TA protein's C-terminal hydrophobic domain serves as the membrane anchor, positioning the bulk of the protein on the cytoplasmic side of the ER membrane.

Question 7

How does the Get1/Get2 complex function as a tail-anchored protein insertase?

Accepted Answer

The Get1/Get2 heterotetrameric complex acts as the membrane insertase for TA proteins. The cytosolic domain of Get2 first captures the Get3-TA protein complex, and this interaction is further stabilized by the Get1 cytosolic domain. Once the Get3-TA protein complex is secured at the membrane, ATP hydrolysis by Get3 triggers the conformational change necessary for the Get1/Get2 complex to insert the TA protein into the ER membrane lipid bilayer.

Tail-anchoring of Proteins in the ER Membrane

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