The Glutaminase Interacting Protein (GIP) is composed of a single PDZ domain that interacts with a growing list of partner proteins, including Glutaminase L, that are involved in a number of cell signaling and cancer pathways. Therefore, GIP makes a good target for structure-based drug design. Here we report the solution structures of both free GIP and GIP bound to the C-terminal peptide analog of Glutaminase L. This is the first reported NMR structure of GIP in a complex with one of its binding partners. Our analysis of both free GIP and GIP complexed with the Glutaminase L peptide provides important insights into how a promiscuous binding domain can have affinity for multiple binding partners. Through a detailed chemical shift perturbation analysis and backbone dynamics studies, we demonstrate here that the binding of the Glutaminase L peptide to GIP is an allosteric event. Taken together, the insights reported here lay the groundwork for the future development of a specific inhibitor for GIP.

A large number of cellular processes are mediated by protein-protein interactions, often specified by particular protein binding modules. PDZ domains are an important class of protein-protein interaction modules that typically bind to the C-terminus of target proteins. These domains act as a scaffold where signaling molecules are linked to a multiprotein complex. Human Glutaminase Interacting Protein (GIP), also known as Tax Interacting Protein, is unique among PDZ domain containing proteins since it is composed almost exclusively of a single PDZ domain rather than one of many domains as part of a larger protein. GIP plays pivotal roles in cellular signaling, protein scaffolding and cancer pathways via its interaction with the C-terminus of a growing list of partner proteins. We have identified novel internal motifs that are recognized by GIP through combinatorial phage library screening. Leu and Asp residues in the consensus sequence were identified to be critical for binding to GIP through site-directed mutagenesis studies. Structure-based models of GIP bound to two different surrogate peptides determined from NMR constraints revealed that the binding pocket is flexible enough to accommodate either the smaller carboxylate (COO−) group of a C-terminal recognition motif or the bulkier aspartate side chain (CH2 COO−) of an internal motif. The non-canonical ILGF loop in GIP moves in for the C-terminal motif but moves out for the internal recognition motifs, allowing binding to different partner proteins. One of the peptides co-localizes with GIP within human glioma cells indicating that GIP might be a potential target for anti-cancer therapeutics.

The vast majority of physiological processes in living cells are mediated by protein-protein interactions often specified by particular protein sequence motifs. PDZ domains, composed of 80–100 amino acid residues, are an important class of interaction motif. Among the PDZ-containing proteins, Glutaminase Interacting Protein (GIP), also known as Tax Interacting Protein TIP-1, is unique in being composed almost exclusively of a single PDZ domain. GIP has important roles in cellular signalling, protein scaffolding and modulation of tumor growth and interact with a number of physiological partner proteins, including Glutaminase L, β-Catenin, FAS, HTLV Tax, HPV E6, Rhotekin and Kir 2.3. To identify the network of proteins that interact with GIP, a human fetal brain cDNA library was screened using a yeast two-hybrid assay with GIP as bait. We identified Brain-specific Angiogenesis Inhibitor 2 (BAI2), a member of the adhesion-G protein-coupled receptors (GPCRs), as a new partner of GIP. BAI2 is expressed primarily in neurons, further expanding GIP cellular functions. The interaction between GIP and the carboxy-terminus of BAI2 was characterized using fluorescence, Circular Dichroism (CD) and Nuclear Magnetic Resonance (NMR) spectroscopy assays. These biophysical analyses support the interaction identified in the yeast two-hybrid assay. This is the first study reporting BAI2 as an interaction partner of GIP.

The Receptor for Advanced Glycation End products (RAGE) is a pattern recognition receptor that signals for inflammation via the NF‐κB pathway. RAGE has been pursued as a potential target to suppress symptoms of diabetes and is of interest in a number of other diseases associated with chronic inflammation, such as inflammatory bowel disease and bronchopulmonary dysplasia. Screening and optimization have previously produced small molecules that inhibit the activity of RAGE in cell‐based assays, but efforts to develop a therapeutically viable direct‐binding RAGE inhibitor have yet to be successful. Here, we show that a fragment‐based approach can be applied to discover fundamentally new types of RAGE inhibitors that specifically target the ligand‐binding surface. A series of systematic assays of structural stability, solubility, and crystallization were performed to select constructs of the RAGE ligand‐binding domain and optimize conditions for NMR‐based screening and co‐crystallization of RAGE with hit fragments. An NMR‐based screen of a highly curated ~14 000‐member fragment library produced 21 fragment leads. Of these, three were selected for elaboration based on structure‐activity relationships generated through cycles of structural analysis by X‐ray crystallography, structure‐guided design principles, and synthetic chemistry. These results, combined with crystal structures of the first linked fragment compounds, demonstrate the applicability of the fragment‐based approach to the discovery of RAGE inhibitors.

Cellular signaling is primarily directed via protein-protein interactions. PDZ (PSD-95/Discs large/ZO-1 homologous) domains are well known protein-protein interaction modules involved in various key signaling pathways. Human Tax-interacting protein 1 (TIP-1), also known as glutaminase interaction protein (GIP), is a Class I PDZ domain protein that recognizes the consensus binding motif X-S/T-X-V/I/L-COOH of the C-terminus of its target proteins. We recently reported that TIP-1 not only interacts via the C-terminus of its target partner proteins but also recognizes an internal motif defined by the consensus sequence S/T-X-V/L-D in the target protein. Identification of new target partners containing either a C-terminal or internal recognition motif has rapidly expanded the TIP-1 protein interaction network. TIP-1 being composed solely of a single PDZ domain is unique among PDZ containing proteins. Since it is involved in many important signaling pathways, it is a possible target for drug design. In this mini review, we have discussed human TIP-1, its structure, mechanism of function, its interactions with target proteins containing different recognition motifs, and its involvement in human diseases. Understanding the molecular mechanisms of TIP-1 interactions with distinct target partners and their role in human diseases will be useful for designing novel therapeutics.