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The immunoglobulin superfamily (IgSF) is a large protein superfamily of membrane and soluble proteins that influence recognition, binding, and adhesion. Among members of this family are cell adhesion molecules (CAMs), which form cell-cell contact points that play key roles in development, cell polarization, and cellular fate. Cadherins (CADs) are calcium-dependent proteins of the adherens junction (AJ), and polarize epithelium and endothelium. The tight junction (TJ) is a multiprotein junctional complex whose function is to control the permeability of the paracellular pathway. At the membrane level, TJs are composed of three types of proteins: claudins (CLDNs), occludin (OCLN) and junctional adhesion molecules (JAMs). JAMs are members of the IgSF while CLDN and OCLN are 4-α-helix membrane proteins. Although JAMs are part of the TJ and reside in the same ultrastructure, they are similar to CADs in their secondary, tertiary, and quaternary protein structure. Crystallographic studies of CADs in the presence of calcium yielded trans interactions that resulted in cell-cell contacts. In the absence of calcium, CADs form cis interactions that do not form cell-cell interactions. The crystal structure of JAM-A, has a quaternary organization of a cis dimer. In spite of the many similarities, a link between CADs and JAMs remains unclear. Beyond this point, the association between JAMs, CLDNs, and OCLN in the TJ is vaguely understood. The JAM family (JAM-A, -B, -C and 4) and their tissue-specific distribution indicate that they are key to understanding the TJ’s function and the interplay with the AJ. JAM-A has been used as a prototype for the other three members of the family, but based on current evidence we hypothesized that these proteins may display unique properties to support TJ’s function in a given tissue. Are JAMs affected by calcium just as CADs? Do CLDNs and OCLN make direct contact with JAMs? Do JAMs coordinate the interplay between TJ and AJ? We designed a strategy based on recombinant proteins and biophysical methods to answer these questions. First, we fused the extracellular domain of each JAM to maltose-binding protein (MBP). Our results indicate that JAM proteins have similar secondary structures, but unique tertiary structures. Surface Plasmon Resonance experiments showed that JAM proteins favored heterotypic compared to homotypic interactions. Second, we addressed the effects of cations (Ca2+, Mg2+, Cu2+, Fe2+, Fe3+, and Zn2+) on JAM-A. The exposure of JAM-A to the resulted in changes in its secondary, tertiary structure, and homotypic binding affinity. Finally, we addressed whether cations had an effect on the other TJ components and if there is an interplay with E-CAD. We determined that in the assembly of a simple TJ and AJ, JAM-A and E-CAD are calcium-dependent, while CLDN1 and OCLN are calcium independent. We conclude that TJ components such as CLDN1 and OCLN may work as anchors to maintain cell-cell interactions while JAM-A and E-CAD would be regulated by cations in order to accommodate other homeostatic functions. |
