Multiple protein interactions involving proposed extracellular loop domains of the tight junction protein occludin

Occludin is a tetraspan integral membrane protein in epithelial and endothelial tight junction (TJ) structures that is projected to have two extracellular loops. We have used peptides emulating central regions of human occludin's first and second loops, termed O-A:101-121 and O-B:210-228, respectively, to examine potential molecular interactions between these two regions of occludin and other TJ proteins. A superficial biophysical assessment of A:101-121 and O-B:210-228 showed them to have dissimilar solution conformation characteristics. Although O-A:101-121 failed to strongly interact with protein components of the human epithelial intestinal cell line T84, O-B:210-228 selectively associated with occludin, claudin-one and the junctional adhesion molecule (JAM)-A. Further, the presence of O-B:210-228, but not O-A:101-121, impeded the recovery of functional TJ structures. A scrambled peptide sequences of O-B:210-228 failed to influence TJ assembly. These studies demonstrate distinct properties for these two extracellular segments of the occludin protein and provide an improved understanding of how specific domains of occludin may interact with proteins present at TJ structures.