Menin interacts with IQGAP1 to enhance intercellular adhesion of beta-cells.

Multiple endocrine neoplasia type 1 (MEN1) is a dominantly inherited tumor syndrome that results from the mutation of the MEN1 gene that encodes protein menin. Stable overexpression of MEN1 has been shown to partially suppress the Ras-mediated morphological changes of fibroblast cells. Little is known about the molecular mechanisms by which menin decreases the oncogenic effects on cell morphology and other phenotypes. Here we showed that ectopic expression of menin in pretumor beta-cells increases islet cell adhesion and reduces cell migration. Our further studies revealed that menin interacts with the scaffold protein, IQ motif containing GTPase activating protein 1 (IQGAP1), reduces GTP-Rac1 interaction with IQGAP1 but increases epithelial cadherin (E-cadherin)/beta-catenin interaction with IQGAP1. Consistent with an essential role for menin in regulating beta-cell adhesion in vivo, accumulations of beta-catenin and E-cadherin are reduced at cell junctions in the islets from Men1-excised mice. Together, these results define a novel menin-IQGAP1 pathway that controls cell migration and cell-cell adhesion in endocrine cells.

Formation of nNOS/PSD-95 PDZ dimer requires a preformed beta-finger structure from the nNOS PDZ domain.

PDZ domains are modular protein units that play important roles in organizing signal transduction complexes. PDZ domains mediate interactions with both C-terminal peptide ligands and other PDZ domains. Here, we used PDZ domains from neuronal nitric oxide synthase (nNOS) and postsynaptic density protein-95 (PSD-95) to explore the mechanism for PDZ-dimer formation. The nNOS PDZ domain terminates with a approximately 30 residue amino acid beta-finger peptide that is shown to be required for nNOS/PSD-95 PDZ dimer formation. In addition, formation of the PDZ dimer requires this beta-finger peptide to be physically anchored to the main body of the canonical nNOS PDZ domain. A buried salt bridge between the beta-finger and the PDZ domain induces and stabilizes the beta-hairpin structure of the nNOS PDZ domain. In apo-nNOS, the beta-finger peptide is partially flexible and adopts a transient beta-strand like structure that is stabilized upon PDZ dimer formation. The flexibility of the NOS PDZ beta-finger is likely to play a critical role in supporting the formation of nNOS/PSD-95 complex. The experimental data also suggest that nNOS PDZ and the second PDZ domain of PSD-95 form a "head-to-tail" dimer similar to the nNOS/syntrophin complex characterized by X-ray crystallography.