30 Current Protein and Peptide Science, 2009, 10, 30-36 1389-2037/09 $55.00+.00 © 2009 Bentham Science Publishers Ltd. The Roles of the PDZ-containing Proteins Bridge-1 and PDZD2 in the Regulation of Insulin Production and Pancreatic Beta-Cell Mass Melissa K. Thomas 1,* , Siu Wai Tsang 3 , Man-Lung Yeung 3 , Po Sing Leung 2 and Kwok-Ming Yao 3,* 1 Laboratory of Molecular Endocrinology and Diabetes Unit, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA; 2 Department of Physiology, Faculty of Medicine and Li Ka Shing Institute of Health Sciences, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, China; and 3 Department of Bio- chemistry, LKS Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, China Abstract: PDZ domains are versatile protein interaction modules with the ability to dimerize and to recognize internal and carboxy-terminal peptide motifs. Their function in mediating the formation of multi-molecular signaling complexes is best understood at neuronal and epithelial membranes. In a screen for interactors that regulate transcription factor function in pancreatic beta cells, we isolated two PDZ-containing proteins Bridge-1 (PSMD9) and PDZD2, which contain one and six PDZ domains, respectively. Here, we review their functions in the regulation of pancreatic beta cells as a nuclear coac- tivator or extracellular signaling molecule. Bridge-1 interacts with both E12 and PDX-1 to stimulate insulin promoter ac- tivity. Recent gain-of-function analysis in both cell and transgenic models has revealed its functions to regulate both insu- lin gene expression and pancreatic beta-cell survival. Little is known about the intracellular function of PDZD2 that is predominantly localized to the endoplasmic reticulum of INS-1E cells. Interestingly, PDZD2 is proteolytically processed by caspase-3 to generate a carboxy-terminal secreted protein (sPDZD2) containing two PDZ domains. Expressed in fetal pancreatic progenitor and INS-1E cells, sPDZD2 when added as recombinant protein exerts concentration-dependent mi- togenic effects on beta-like cells. We propose that the PDZ domain proteins Bridge-1 and PDZD2 likely transduce signals that regulate insulin production, proliferation, and survival of pancreatic beta cells. Keywords: PDZ, Bridge-1, PDZD2, insulin, pancreas. PDZ DOMAINS ARE ABUNDANTLY PRESENT IN METAZOAN SIGNALING PROTEINS The name PDZ was derived from the first three proteins [P SD-95 (a 95-kDa signaling protein in post-synaptic den- sity), D lg (Drosophila discs large protein) and Z O1 (zonula occludens 1 protein)] that were identified to contain PDZ domains [1, 2]. Since PDZ domains possess repeats of the highly conserved G-L-G-F sequence, they are also called GLGF repeats or DHR (Discs large homology repeat) do- mains. PDZ domains are conserved regions of about 90 amino acids comprising 6 -strands (A-F) and 2 -helixes (A and B) compactly that are arranged in a globular structure [3]. The binding to a ligand takes place in an elongated sur- face groove where the anti-parallel -strand interacts with the B strand and the B helix. In general, PDZ domains are found in diverse signaling proteins in bacteria, yeasts, plants, insects and vertebrates (reviewed in [4-6]). The high propor- tion of PDZ domains in metazoans indicates that they may have evolved in response to the increased signaling needs of multicellular organisms. In the human and mouse genomes, PDZ domains are more abundant than SH2 or SH3 domains [7] and PDZ domain was ranked 19th among the most abun- *Address correspondence to these authors at the Department of Biochemis- try, LKS Faculty of Medicine, The University of Hong Kong, 3/F Labora- tory Block, LKS Faculty of Medicine Building, 21 Sassoon Road, Pokfu- lam, Hong Kong SAR, China; E-mail: kmyao@hkusua.hku.hk and Labora- tory of Molecular Endocrinology and Diabetes Unit, Massachusetts General Hospital, Thier 340, 50 Blossom Street, Boston, Massachusetts 02114, USA; E-mail: mthomas1@partners.org dant domains identified in the first draft of the human ge- nome [8]. The function of PDZ domains is to mediate protein- protein interactions by binding in a sequence-specific fash- ion to short carboxy-terminal peptides or internal peptides that fold in a -hairpin [4, 5, 9]. In addition, PDZ domains can interact with greater versatility through PDZ-PDZ do- main interactions. Attempts have been made to classify PDZ domains based on their binding specificities [4, 9-12] but no simple classification accommodates the wide range of selec- tivity and affinity recently revealed in large-scale binding studies [13, 14]. These recent analyses underscore the broad binding specificity and adaptive plasticity of PDZ domains. The PDZ domains often are found in multiple copies and in combination with other protein interaction domains to facilitate signaling, establish cell polarity and direct protein trafficking. These domains play important roles in transport, localization and assembly of supramolecular signaling com- plexes. This scaffolding function is well characterized in synapses and polarized epithelial cells [6, 15, 16]. It is be- lieved that some PDZ-containing proteins are more dynami- cally regulated in distribution and may also be involved in the trafficking of interacting proteins within the cell. Al- though PDZ domain-mediated signaling likely influences a range of cellular functions within the insulin-producing beta cells of the pancreas, in this review we focus on two novel PDZ domain-containing proteins Bridge-1 and PDZD2 that were identified in a yeast two-hybrid screen for interactions with the basic helix-loop-helix transcription factor E12 [17].