Wnt/ß-catenin signaling requires interaction of the Dishevelled DEP domain and C terminus with a discontinuous motif in Frizzled.

Wnt binding to members of the seven-span transmembrane Frizzled (Fz) receptor family controls essential cell fate decisions and tissue polarity during development and in adulthood. The Fz-mediated membrane recruitment of the cytoplasmic effector Dishevelled (Dvl) is a critical step in Wnt/ß-catenin signaling initiation, but how Fz and Dvl act together to drive downstream signaling events remains largely undefined. Here, we use an Fz peptide-based microarray to uncover a mechanistically important role of the bipartite Dvl DEP domain and C terminal region (DEP-C) in binding a three-segmented discontinuous motif in Fz. We show that cooperative use of two conserved motifs in the third intracellular loop and the classic C-terminal motif of Fz is required for DEP-C binding and Wnt-induced ß-catenin activation in cultured cells and Xenopus embryos. Within the complex, the Dvl DEP domain mainly binds the Fz C-terminal tail, whereas a short region at the Dvl C-terminal end is required to bind the Fz third loop and stabilize the Fz-Dvl interaction. We conclude that Dvl DEP-C binding to Fz is a key event in Wnt-mediated signaling relay to ß-catenin. The discontinuous nature of the Fz-Dvl interface may allow for precise regulation of the interaction in the control of Wnt-dependent cellular responses.

The Cohesin Complex Is Necessary for Epidermal Progenitor Cell Function through Maintenance of Self-Renewal Genes.

Adult stem and progenitor cells are critical for replenishing lost tissue due to injury or normal turnover. How these cells maintain self-renewal and sustain the tissue they populate are areas of active investigation. Here, we show that the cohesin complex, which has previously been implicated in regulating chromosome segregation and gene expression, is necessary to promote epidermal stem and progenitor cell self-renewal through cell-autonomous mechanisms. Cohesin binds to genomic sites associated with open chromatin, including DNase-I-hypersensitive sites, RNA polymerase II, and histone marks such as H3K27ac and H3K4me3. Reduced cohesin expression results in spontaneous epidermal differentiation due to loss of open chromatin structure and expression of key self-renewal genes. Our results demonstrate a prominent role for cohesin in modulating chromatin structure to allow for enforcement of a stem and progenitor cell gene expression program.

Stochastic machines as a colocalization mechanism for scaffold protein function.

The axis inhibition (Axin) scaffold protein colocalizes ß-catenin, casein kinase Iα, and glycogen synthetase kinase 3ß by their binding to Axin's long intrinsically disordered region, thereby yielding structured domains with flexible linkers. This complex leads to the phosphorylation of ß-catenin, marking it for destruction. Fusing proteins with flexible linkers vastly accelerates chemical interactions between them by their colocalization. Here we propose that the complex works by random movements of a "stochastic machine," not by coordinated conformational changes. This non-covalent, modular assembly process allows the various molecular machine components to be used in multiple processes.

Critical scaffolding regions of the tumor suppressor Axin1 are natively unfolded.

The Wnt pathway tumor-suppressor protein Axin coordinates the formation of a critical multiprotein destruction complex that serves to downregulate ß-catenin protein levels, thereby preventing target gene activation. Given the lack of structural information on some of the major functional parts of Axin, it remains unresolved how the recruitment and positioning of Wnt pathway kinases, such as glycogen synthase kinase 3ß, are coordinated to bring about ß-catenin phosphorylation. Using various biochemical and biophysical methods, we demonstrate here that the central region of Axin that is implicated in binding glycogen synthase kinase 3ß and ß-catenin is natively unfolded. Our results support a model in which the unfolded nature of these critical scaffolding regions in Axin facilitates dynamic interactions with a kinase and its substrate, which in turn act upon each other.