Identification of amino acids essential for angulin-1/3 binding of the tricellular tight junction binder, angubindin-1.

Tight junctions (TJs) are formed where two or three cells meet and are therefore categorized, respectively, into bicellular TJs (bTJs) and tricellular TJs (tTJs). Angubindin-1 is the first tTJ modulator enhancing intestinal macromolecule permeation via binding to the key tTJ proteins, angulin-1 and angulin-3. It is a fragment (amino acids 421-664) derived from domain IV of Clostridium perfringens iota toxin. Here, we identified critical residues (L562, L598, E638, V640, Y643, K644) of angubindin-1 to be essential for binding to angulins by alanine scanning. Mutants substituting these amino acids with alanine exhibited reduced binding to angulin-expressing cells. Simultaneous substitution of all these amino acids lost binding to angulins and resulted in the loss of tTJ-modulating functions of angubindin-1. These insights highlight crucial residues for the tTJ-modulating activity of angubindin-1, which may hold promise in the design of noninvasive, targeted therapeutics using angubindin-1 as a prototype tTJ modulator to enhance the permeation of drugs.