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?

1) By forming hydrogen bonds with the membrane
2) By interacting with phospholipids in the membrane
3) By undergoing conformational changes upon membrane binding
4) By covalently attaching to lipids in the membrane

Answer 1

Option 4.

The C-terminal sequence attaches these proteins to the membrane most likely through covalent attachment to lipids, facilitated by the cysteine residue at the end that can form bonds such as palmitoylation or prenylation.

Step-by-step explanation:

The amino acid sequence at the C-terminus of the membrane proteins described (-KKKKKXXC) suggests that these proteins are most likely attached to the membrane by covalent attachment to lipids in the membrane.

The presence of multiple lysine residues (K) indicates a region that is highly positive charged, which can facilitate interactions with negatively charged groups on the phospholipid head groups, or form electrostatic interactions with other proteins or ions in the vicinity of the membrane.

However, it is the cysteine (C) at the end of the sequence that is known to form covalent bonds with fatty acid chains, such as through a process known as post-translational modification, specifically palmitoylation or prenylation.

These modifications involve the addition of lipid groups to the cysteine residue, anchoring the protein strongly to the membrane.