Question

1. You are studying an integral membrane protein of the plasma membrane. It has a single transmembrane domain and the N-terminal portion of this protein is extracellular. You decide to perform mutagenesis studies of the stop-transfer sequence, and you mutate all of the valine and isoleucine residues within this sequence to lysine and arginine residues. What do you predict will happen to the transport of this protein and explain why

Answer 1

A 22 to 25 amino acid sequence present in the central section of the protein, which gives rise to an alpha helix in the membrane is known as the stop-transfer anchor sequence. The sequence plays an essential function in targeting the protein towards the plasma membrane. On the other hand, it also ceases targeting of the protein towards the endoplasmic reticulum, which was started by the signal peptide.

Thus, the process of translation of the remaining of the protein occurs within the cytosol due to the tethering of the transmembrane domain. In the stop-transfer anchor sequence, the hydrophobic amino acids present are isoleucine and valine. After mutation, these amino acids get converted into arginine and lysine, thus, hydrophilic amino acids replace hydrophobic amino acids in the sequence.

Due to this, the transmembrane domain cannot be targeted towards an integral part of the plasma membrane by the short transfer anchor sequence, and therefore, now the translocation of the protein will take place towards the endoplasmic reticulum as initiated by the signal peptide at the beginning.