Tyrosine phosphorylation of occludin attenuates its interactions with ZO-1, ZO-2, and ZO-3

Occludin, the transmembrane integral protein of the tight junction, plays a crucial role in the molecular organization and function of tight junction. While the homotypic interaction of extracellular loops of occludin appears to determine the barrier function of tight junction, the intracellular C-terminal tail, C-occludin, interacts with other tight junction proteins such as ZO-1, ZO-2, and ZO-3 and with the actin filaments of cytoskeleton. In the present study we phosphorylated GST-fused C-occludin on tyrosine residues, in TKX1 Epicurian coli or by active c-Src in vitro. c-Src binds to occludin and phosphorylates it on tyrosine residues. The effect of tyrosine phosphorylation of C-occludin on its ability to bind ZO-1, ZO-2, ZO-3, and F-actin was evaluated. Results show that the amounts of ZO-1, ZO-2, and ZO-3 bound to tyrosine phosphorylated C-occludin were several fold less than the amounts bound to non-phosphorylated C-occludin. However, the amount of tyrosine phosphorylated C-occludin bound to F-actin was not significantly different from the amount of non-phosphorylated C-occludin bound to F-actin. These results demonstrate that tyrosine phosphorylation of occludin reduces its ability to bind ZO-1, ZO-2, and ZO-3, but not F-actin. Results also suggest that c-Src-mediated disruption of tight junction may involve tyrosine phosphorylation of occludin.