Which statement best characterizes ZO proteins as described in the material?

• A. They are transmembrane receptors.
• B. They are cytosolic enzymes.
• C. They are PDZ domain proteins.
• D. They are lipoproteins.

Answer: (C)

ZO proteins act as scaffold components of tight junctions, located on the cytoplasmic side where they organize and stabilize the junction by connecting transmembrane junctional proteins to the actin cytoskeleton. A key feature is that they contain PDZ domains, protein-interaction modules that bind the C-termini of transmembrane partners like claudins and occludin. This makes them PDZ domain proteins, which is why that option best describes them. They are not transmembrane receptors (they don’t span the membrane), not enzymes (they don’t catalyze reactions), and not lipoproteins (they are proteins, not lipid-associated carriers).