A phosphorylation hotspot within the occludin C-terminal domain.

Tight junctions (TJs) form paracellular barriers defining the permeability characteristics of epithelial and endothelial cell layers in our body. Tetraspanin integral membrane proteins, including occludin, tricellulin, MarvelD3, and a set of claudins, form a network of anastomosing strands bringing the membranes of neighboring cells into close contact. Occludin is assumed to play an important role in the regulation of TJ formation, structure, and function, and is tightly regulated by phosphorylation. We here summarize the role of occludin phosphorylation on assembly/disassembly and function of TJs and specifically focus on a cluster of 11 amino acids in the C-terminal cytoplasmic domain of occludin (Tyr398-Ser408), including highly conserved phosphorylation sites for c-Src, PKCs, and CK2. Phosphorylation by these kinases affects occludin localization, dynamics, and interaction with other TJ proteins. Interestingly, this phosphorylation hotspot is localized in an unstructured region close to the ZO-1 binding site, and a cysteine residue which is involved in intermolecular disulfide-bond formation thus contributing to occludin dimerization. We discuss potential consequences and open questions in respect to the functional role of this phosphorylation hotspot.