Which of the following experimental data would suggest the presence of a transmembrane protein?

• A. A protein is easily removed from the membrane by washing with a mild buffer.
• B. A protein is released from the membrane after treatment with low concentrations of urea.
• C. A protein remains associated with the membrane after treatment with high-salt solutions but is released by detergents.
• D. A protein is found only in the cytosol and not associated with any membrane fraction.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of transmembrane proteins. Transmembrane proteins span the lipid bilayer and are embedded within the membrane. They typically have hydrophobic regions that interact with the lipid bilayer and hydrophilic regions that extend into the aqueous environment on either side of the membrane.
2. Step 2: Review the experimental methods mentioned in the problem. Mild buffer washing, low concentrations of urea, high-salt solutions, and detergents are techniques used to study protein-membrane associations. Each method has a different mechanism for disrupting protein interactions with membranes.
3. Step 3: Analyze the experimental data provided. Proteins that are easily removed by mild buffers or low concentrations of urea are likely peripheral proteins, which are loosely associated with the membrane. Proteins that remain associated with the membrane after treatment with high-salt solutions but are released by detergents are likely transmembrane proteins, as detergents disrupt the lipid bilayer and solubilize hydrophobic regions. Proteins found only in the cytosol are not membrane-associated.
4. Step 4: Focus on the key characteristic of transmembrane proteins. Transmembrane proteins are strongly embedded in the membrane and require detergents to disrupt the lipid bilayer for their release. This distinguishes them from peripheral proteins, which are more loosely attached and can be removed by milder treatments.
5. Step 5: Conclude that the experimental data indicating a protein remains associated with the membrane after treatment with high-salt solutions but is released by detergents suggests the presence of a transmembrane protein. This is because detergents are specifically required to solubilize the hydrophobic regions of transmembrane proteins.