0 The major focus of this review is on the molecular structure and functions of proteins that are involved in the formation of tight intercellular junctions and some methods used to modulate intercellular junctions. Tight junctions are a barricade for paracellular delivery of molecules such as peptides and proteins. Understanding the molecular structure and functions of intercellular junction proteins may provide the possibility of systematically designing molecules that can regulate tight junctions. Regulation of proteins involved in the formation of intercellular junctions may open avenues to modulating the porosity of intercellular junctions to improve paracellular drug delivery.

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Cadherins are calcium-binding proteins which are responsible for cell-cell adhesion in biological systems. Cadherins are involved in embryo compaction, neurite growth, cellular differentiation and formation of biological barriers (i.e., intestinal and blood brain barriers). A short linear peptide, LRAHAVDVNG-NH2 (Peptide 1), which contains His-Ala-Val sequence and is derived from the N-cadherin sequence has been shown to inhibit embryo compaction and neurite growth; this is caused by inhibition of the cadherin-cadherin interactions. Peptide 1 was synthesized and its solution conformation was determined by proton nuclear magnetic resonance, circular dichroism and molecular dynamics simulations. These studies indicated that the peptide has an extended structure from residue Leu1 to Asp7, possibly a beta-sheet structure, followed by a beta-turn from Asp7 to Gly10. The X-ray crystal structure of a sequence similar to that of peptide 1 in hemagglutinin indicated that it also has a beta-sheet structure around the HAV sequence, followed by a beta-turn.

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