Claudins are the critical transmembrane proteins in tight junctions. Claudin-5, for instance, prevents paracellular permeation of small molecules. However, the molecular interaction mechanism is unknown. Hence, the claudin-claudin interaction and tight junction strand formation were investigated using systematic single mutations. Claudin-5 mutants transfected into tight junction-free cells demonstrated that the extracellular loop 2 is involved in strand formation via trans-interaction, but not via polymerization, along the plasma membrane of one cell. Three phenotypes were obtained: the tight junction type (wild-type-like trans- and cis-interaction; the disjunction type (blocked trans-interaction); the intracellular type (disturbed folding). Combining site-directed mutagenesis, live-cell imaging-, electron microscopy-, and molecular modeling data led to an antiparallel homodimer homology model of the loop. These data for the first time explain how two claudins hold onto each other and constrict the paracellular space. The intermolecular interface includes aromatic (F147, Y148, Y158) and hydrophilic (Q156, E159) residues. The aromatic residues form a strong binding core between two loops from opposing cells. Since nearly all these residues are conserved in most claudins, our findings are of general relevance for all classical claudins. On the basis of the data we have established a novel molecular concept for tight junction formation.

One hypothesis to explain how mutations in the same nuclear envelope proteins yield pathologies focused in distinct tissues is that as yet unidentified tissue-specific partners mediate the disease pathologies. The nuclear envelope proteome was recently determined from leukocytes and muscle. Here the same methodology is applied to liver and a direct comparison of the liver, muscle and leukocyte data sets is presented. At least 74 novel transmembrane proteins identified in these studies have been directly confirmed at the nuclear envelope. Within this set, RT-PCR, western blot and staining of tissue cryosections confirms that the protein complement of the nuclear envelope is clearly distinct from one tissue to another. Bioinformatics reveals similar divergence between tissues across the larger data sets. For proteins acting in complexes according to interactome data, the whole complex often exhibited the same tissue-specificity. Other tissue-specific nuclear envelope proteins identified were known proteins with functions in signaling and gene regulation. The high tissue specificity in the nuclear envelope likely underlies the complex disease pathologies and argues that all organelle proteomes warrant re-examination in multiple tissues.