| Zonula Occludens-1 (ZO-1) is a membrane-associated guanylate kinase-like (MAGUK) protein. The core module, composed of three consecutive domains Src Homology 3-Unique 5-Guanylate kinase (SH3-U5-GUK), is the minimal requirement for membrane localization and tight junction (TJ) formation.;We have solved the structure of this core module to 2.6 A and observed the conservation of the intramolecular beta-beta (beta-beta) strand unit that completes the SH3 fold as predicted by a previous structure of the homologous post-synaptic density-95 (PSD-95) MAGUK core module. Interestingly, interdomain-angle differences between the SH3 and GUK domains of the three MAGUK proteins compared --- ZO-1 with ZO3 and PSD-95 --- were observed with the ZO-1 core module adopting a more open, and a more closed conformation compared to PSD-95 and ZO-3 respectively. These conformational differences between the different MAGUKs likely reflect a general variation within the core modules of MAGUKs that could have functional and regulatory implications.;We have also made the novel discovery that the unique 6 (U6) region, composed of a stretch of acidic residues immediately C-terminal to the GUK domain, directly binds to the core module, a reaction which is bivalent-cation dependent. As the U6 domain and the calcium sensor calmodulin both compete for similar binding sites, we propose that this direct binding interaction between U6 and the core module may be one general mechanism by which U6 regulates core module function. |
