The structural and functional determinants of the Axin and Dishevelled DIX domains

An evolutionary trace analysis of DIX domains identified conserved residues which, when mapped onto the crystal structure of the Axin DIX domain and a comparative model of the Dishevelled DIX domain, allow their categorization as residues of either structural or functional importance. We identify residues that are structural and functional determinants of the DIX domain fold, as well as those that are specific to homodimerization of Axin and Dishevelled.This report provides the first explanation of the mutant phenotypes caused by non-synonymous substitutions in the Dishevelled and Axin DIX domain by correlating their presumed functional significance with molecular structure.Signalling by members of the Wnt family of secreted glycoproteins relies on dynamic interactions between protein complexes to regulate transcription, the cytoskeleton and cell adhesion [1,2]. Binding of Wnt proteins to their receptors, members of the Frizzled and LRP families, leads to the activation of Dishevelled, a multi-domain protein that mediates many functions of Wnt [1-3] and this triggers a sequence of protein-protein interactions, which culminate in specific effector activities.In the 'canonical' pathway, Wnt proteins regulate transcription of specific genes by modulating the amount, activity and intracellular location of β-catenin, a multi-domain protein with a role in linking Cadherins to the cytoskeleton, in addition to being an effector of Wnt signalling [4]. In the absence of Wnt, a soluble pool of β-catenin is targeted for degradation by a multiprotein complex assembled around the scaffolding protein Axin and in which glycogen synthase kinase 3β (GSK3β) is the catalytic component. GSK3β phosphorylates β-catenin, thus targeting it for degradation by the proteosome [1,4]. Upon Wnt signalling, Dishevelled prevents β-catenin degradation by interfering with the interaction of the Axin-APC-GSK3β complex and β-catenin [1,5-8]. This activity involves a direct interaction between Dishevell