Question

The following diagram depicts the topology of a multipass transmembrane protein in the endoplasmic reticulum (ER) membrane. Which set of helices act as stop-transfer signals in this protein?

1) 1, 2, 3
2) 4, 5, 6
3) 1, 3, 5
4) 2, 4, 6
5) 1, 6

Answer 1

The helices that act as stop-transfer signals in the topology of a multipass transmembrane protein within the ER membrane are helices 1 and 6. These signals prevent the protein from traversing the entire membrane and ensure the correct positioning within the lipid bilayer.

Step-by-step explanation:

In the topology of a multipass transmembrane protein within the endoplasmic reticulum (ER) membrane, the helices that act as stop-transfer signals are crucial because they prevent the protein from passing completely through the ER membrane. These hydrophobic domains bind the protein in the lipid interior of the membrane. The correct set of helices that serve as stop-transfer signals in the depicted protein are helices 1 and 6.

During the synthesis of integral membrane proteins, the interaction of signal recognition particles (SRPs) with ribosomes and the hydrophobic signal peptides directs them to the ER. Subsequent polypeptide elongation halts when these sequences act as stop-transfer signals. As integral membrane proteins may span the membrane multiple times, it is these stop-transfer sequences that enable the accurate insertion and orientation of these proteins within the membrane. The alpha-helical domains, rich in hydrophobic amino acids, anchor the proteins in the membrane due to their non-polar nature.