Question

33. A group of membrane proteins, which can be extracted only from membranes using detergents, are all found to have a similar amino acid sequence at their carboxyl 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 carboxyl-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.
amphipathic

hydrophobic

phosphatidylserine
cholesterol

Lipid bilayer

phospholipids
Fatty acid tails

Lipid monolayer

proteins
glycolipids

lipids

sterols
Hydrophilic head groups

phosphatidylcholine


sugars

(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 positively charged lysine residues interact with negatively charged phospholipids like phosphatidylserine.

Answer 1

The carboxyl-terminal sequence attaches membrane proteins to the membrane through a covalent bond with a membrane lipid, typically via a cysteine residue forming a bond with a fatty acid tail. This is a common posttranslational modification for anchoring proteins to membranes.

Step-by-step explanation:

The most likely way in which the carboxyl-terminal sequence attaches membrane proteins to the membrane is through a covalent bond with a membrane lipid. Specifically, the cysteine residue (-KKKKKXXC) at the carboxyl terminus can form a thioester bond with a lipid molecule, such as a fatty acid tail, through a process called posttranslational modification. This allows the protein to be anchored to the membrane's hydrophobic interior.

Membrane proteins, especially integral proteins, have hydrophobic regions that allow them to be embedded within the membrane. These proteins can span the bilayer with their hydrophobic regions interacting with the lipid tails, whereas the hydrophilic domains interact with the cytosol or the extracellular environment. The sequence provided is indicative of a lipid anchoring motif, commonly found at the end of some membrane proteins, serving as a mechanism for membrane association and proper localization of the protein.