Exploration of the Structure and Function of ZU5 Domains.

摘要

ZU5 domain was first identified in the cytoplasmic domains of the tight junction protein ZO-l and Unc5-like netrin receptors (hence the domain name ZU5). It is a novel protein domain of unknown function. ZU5 domains are also present in different forms of ankyrins and apoptosis protein PIDD. The cytoplasmic portion of UNC5 is responsible for netrin-mediated signaling events in axonal migrations, blood vessel patterning, and apoptosis, although the molecular mechanisms governing these processes are unknown. To provide a foundation for elucidating UNC5-mediated signaling mechanism, we determined the crystal structure of the cytoplasmic portion of UNC5b. The structure shows that the cytoplasmic portion of UNC5b contains three distinctly folded domains, namely ZU5, UPA, and death domain (DD). These three domains form a structural supramodule with ZU5 binding to both UPA and DD and thereby maintaining the ZU5-UPA-DD supramodule in a closed conformation with suppressed biological activities. Release of the closed conformation of the ZU5-UPA-DD supramodule led to the activation of the protein assayed in the context of apoptosis and blood vessel patterning. Finally, we provide evidence showing that the supramodular nature of UNC5 ZU5-UPD-DD is shared by the ankyrin and PIDD families of scaffold proteins. Another ZU5 domain-containing protein Zonula occludens (ZO)-l is a multi-domain scaffold protein that plays critical roles in tight junction formation by targeting, anchoring, and clustering transmembrane adhesion molecules and cytoskeletons. ZO-l is unique among the ZO family members, as it contains an additional ZU5 domain at its extreme C-terminus. This ZU5 domain has been implicated to be required for the proper localization of ZO-l to tight junctions, although the molecular mechanism governing this process is unknown. In this work, we discovered that ZO-l ZU5 functions as a novel protein-protein interaction domain. The molecular basis of ZO-l ZU5-mediated target recognition was elucidated by solving the solution structures of the ZU5 domain in its apo- and target-bound forms. We found that the ZO-l ZU5 domain in its apo-form adopts a partial ZU5-fold lacking several beta-strands. The target peptide binds to ZO-l ZU5 in a beta-sheet conformation and complements the partial ZU5 domain into a complete ZU5-fold. We provide evidence that the ZU5 domain plays a regulatory role in the localization of ZO-l at tight junctions.