Which proteins are PDZ domain proteins involved in epithelial junctions?

• A. Occludin, Claudin, E-cadherin
• B. ZO-1, ZO-2, ZO-3
• C. Integrin, Fibronectin, Laminin
• D. Mucin, Keratin, Desmoglein

Answer: (B)

PDZ domain proteins act as scaffolds that organize tight junction components in epithelia. Zonula occludens proteins—ZO-1, ZO-2, and ZO-3—are classic PDZ-domain–containing scaffolds at tight junctions. Their PDZ domains bind the C-terminal tails of transmembrane tight junction proteins like occludin and claudins, helping assemble the junction and link them to the actin cytoskeleton. This scaffolding is crucial for the tight seal that controls paracellular permeability and for coordinating junctional complex formation during epithelial barriers. Other options list proteins that are not PDZ-domain scaffolds at tight junctions: occludin and claudins are the transmembrane components themselves; E-cadherin is part of adherens junctions; integrins, fibronectin, and laminin relate to the extracellular matrix; mucin, keratin, and desmoglein are not PDZ-domain junction scaffolds.