Where are PDZ domain proteins found?

• A. Zonula Occludens
• B. Zonula Adherens
• C. Macula Adherens
• D. Gap junctions

Answer: (A)

PDZ domains act as modular protein binding sites that organize complexes at cell–cell junctions, especially at tight junctions. In epithelial cells, the tight junction scaffolding proteins Zonula Occludens (ZO-1, ZO-2, ZO-3) contain PDZ domains that bind the cytoplasmic tails of transmembrane tight-junction proteins like occludin and claudins and link them to the actin cytoskeleton. This arrangement creates the seal between neighboring cells that defines a Zonula Occludens. In contrast, adherens junctions are cadherin-based and connect to actin via catenins, desmosomes (macula adherens) link to intermediate filaments, and gap junctions form channels made of connexins. So PDZ domain proteins are best associated with Zonula Occludens.