Which statement best defines an integral membrane protein?

• A. A protein permanently associated with the lipid bilayer that penetrates the hydrophobic core and typically requires detergents to be removed
• B. A protein that binds transiently to the membrane surface through ionic interactions and can be released by high salt or pH changes
• C. A soluble cytosolic protein that never associates with membranes and remains in the aqueous phase after cell fractionation
• D. A protein attached to the membrane only by a covalently linked carbohydrate chain on the extracellular surface

Answer: (A)

Verified step by step guidance

1. Understand the definition of integral membrane proteins: these proteins are permanently embedded within the lipid bilayer of the membrane.
2. Recognize that integral membrane proteins penetrate the hydrophobic core of the lipid bilayer, which means they interact extensively with the nonpolar interior of the membrane.
3. Note that because of their strong hydrophobic interactions, integral membrane proteins typically require detergents (amphipathic molecules) to be solubilized and removed from the membrane.
4. Contrast integral membrane proteins with peripheral membrane proteins, which associate transiently with the membrane surface and can be released by changes in ionic strength or pH.
5. Identify that proteins attached only by covalently linked carbohydrate chains or soluble cytosolic proteins do not penetrate the hydrophobic core and thus are not integral membrane proteins.