The PHCCEx domain of Tiam1/2 is a novel protein- and membrane-binding module

Tiam1 and Tiam2 (Tiam1/2) are guanine nucleotide-exchange factors that possess the PH–CC–Ex (pleckstrin homology, coiled coil and extra) region that mediates binding to plasma membranes and signalling proteins in the activation of Rac GTPases. Crystal structures of the PH–CC–Ex regions revealed a single globular domain, PHCCEx domain, comprising a conventional PH subdomain associated with an antiparallel coiled coil of CC subdomain and a novel three-helical globular Ex subdomain. The PH subdomain resembles the β-spectrin PH domain, suggesting non-canonical phosphatidylinositol binding. Mutational and binding studies indicated that CC and Ex subdomains form a positively charged surface for protein binding. We identified two unique acidic sequence motifs in Tiam1/2-interacting proteins for binding to PHCCEx domain, Motif-I in CD44 and ephrinB's and the NMDA receptor, and Motif-II in Par3 and JIP2. Our results suggest the molecular basis by which the Tiam1/2 PHCCEx domain facilitates dual binding to membranes and signalling proteins

Structural basis for Ccd1 auto-inhibition in the Wnt pathway through homomerization of the DIX domain

Wnt signaling plays an important role in governing cell fate decisions. Coiled-coil-DIX1 (Ccd1), Dishevelled (Dvl), and Axin are signaling proteins that regulate the canonical pathway by controlling the stability of a key signal transducer β-catenin. These proteins contain the DIX domain with a ubiquitin-like fold, which mediates their interaction in the β-catenin destruction complex through dynamic head-to-tail polymerization. Despite high sequence similarities, mammalian Ccd1 shows weaker stimulation of β-catenin transcriptional activity compared with zebrafish (z) Ccd1 in cultured cells. Here, we show that the mouse (m) Ccd1 DIX domain displays weaker ability for homopolymerization than that of zCcd1. Furthermore, X-ray crystallographic analysis of mCcd1 and zCcd1 DIX domains revealed that mCcd1 was assembled into a double-helical filament by the insertion of the β1-β2 loop into the head-to-tail interface, whereas zCcd1 formed a typical single-helical polymer similar to Dvl1 and Axin. The mutation in the contact interface of mCcd1 double-helical polymer changed the hydrodynamic properties of mCcd1 so that it acquired the ability to induce Wnt-specific transcriptional activity similar to zCcd1. These findings suggest a novel regulatory mechanism by which mCcd1 modulates Wnt signaling through auto-inhibition of dynamic head-to-tail homopolymerization

Crystallographic characterization of the membrane-targeting domains of the Rac-specific guanine nucleotide-exchange factors Tiam1 and Tiam2

The membrane-targeting domains of the Rac-specific guanine nucleotide-exchange factors Tiam1 and Tiam2 were purified and crystallized

Crystallographic characterization of the radixin FERM domain bound to the cytoplasmic tail of membrane-type 1 matrix metalloproteinase (MT1-MMP)

The radixin FERM domain was shown to bind the MT1-MMP cytoplasmic peptide and crystals of the complex were obtained

Crystallographic characterization of the radixin FERM domain bound to the cytoplasmic tail of adhesion molecule CD44

The radixin FERM domain complexed with the CD44 cytoplasmic tail peptide has been crystallized. A diffraction data set from the complex was collected to 2.1 Å

Crystallographic characterization of the radixin FERM domain bound to the cytoplasmic tails of adhesion molecules CD43 and PSGL-1

The radixin FERM domain has been crystallized in complex with CD43 and PSGL-1 peptides. Diffraction data sets were collected from the complexes to 2.9 and 2.8 Å resolution, respectively