AUTHOR=Ahlswede Lena , Siebenaller Carmen , Junglas Benedikt , Hellmann Nadja , Schneider Dirk 

TITLE=Human Claudin-7 cis-Interactions Are Not Crucial for Membrane-Membrane (Trans-) Interactions

JOURNAL=Frontiers in Molecular Biosciences

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/molecular-biosciences/articles/10.3389/fmolb.2022.908383

DOI=10.3389/fmolb.2022.908383

ISSN=2296-889X

ABSTRACT=<p>Human Claudin-7 (Cldn7) is a member of the Claudin (Cldn) superfamily. <italic>In vivo</italic>, these proteins form tight junctions, which establish constricted connections between cells. Cldns oligomerize within the membrane plane (= <italic>cis</italic>-interaction), and also interact with Cldns from adjacent cells (= <italic>trans</italic>-interaction). Interactions of Cldns are typically studied <italic>in vivo</italic> and structural analyses of isolated Cldns are limited. Here, we describe heterologous expression in <italic>E. coli</italic> and purification of human Cldn7, enabling <italic>in vitro</italic> analyses of the isolated protein using detergent and model membrane systems. Cldn7 exists as a monomer, hexamer, and various higher oligomers in micelles. While only limited unfolding of the protein was observed in the presence of the anionic detergent sodium dodecyl sulfate, decreased ionic strength did affect Cldn7 <italic>cis</italic>-interactions. Furthermore, we identified two amino acids which mediate electrostatic <italic>cis</italic>-interactions and analyzed the impact of disturbed <italic>cis</italic>-interaction on <italic>trans</italic>-contacts via atomic force microscopy and monitoring Förster resonance energy transfer between fluorescently labeled Cldn7-containing proteoliposomes. Our results indicate that Cldn7 <italic>cis</italic>-oligomerization might not be a prerequisite for establishing <italic>trans</italic>-contacts.</p>