Question

A group of membrane proteins can be extracted from membranes only by using detergents. All the

proteins in this group have a similar amino acid sequence at their C-terminus: -KKKKKXXC (where K
stands for lysine, X stands for any amino acid, and C stands for cysteine). This sequence is essential
for their attachment to the membrane. What is the most likely way in which the C-terminal sequence
attaches these proteins to the membrane?
A) The cysteine residue is covalently attached to a membrane lipid.
B) The peptide spans the membrane as an α helix.
C) The peptide spans the membrane as part of a β sheet.
D) The positively charged lysine residues interact with an acidic integral membrane protein.
E) The XX residues interact with a basic integral membrane protein.

Answer 1

The most likely way in which the C-terminal sequence -KKKKKXXC attaches these proteins to the membrane is through a covalent attachment of the cysteine residue to a membrane lipid.

Step-by-step explanation:

The sequence at the C-terminus of the proteins, -KKKKKXXC, suggests that the mode of attachment to the membrane is through a covalent bond between the cysteine residue and a lipid in the membrane.

This is because the cysteine residue has a thiol group (-SH) that is known to form covalent disulfide bonds with other thiol groups or can be post-translationally modified to carry a lipid group, thus anchoring the protein to the membrane.

The sequence does not suggest that the protein spans the membrane as an α helix or as part of a β sheet, and although lysine residues are positively charged, they are not the primary mode of attachment in this case. The XX residues are unspecified and do not provide a strong indication of their role in membrane attachment, making option (E) less likely.