The Na+ -H + exchanger regulatory factor (NHE-RF) is a cytoplasmic phosphoprotein that was first found to be involved in protein kinase A mediated regulation of ion transport. NHE-RF contains two distinct protein interaction PDZ domains: NHE-RF-PDZ1 and NHE-RF-PDZ2. However, their binding partners are currently unknown. Because PDZ domains usually bind to specific short linear C-terminal sequences, we have carried out affinity selection of random peptides for specific sequences that interact with the NHE-RF PDZ domains and found that NHE-RF-PDZ1 is capable of binding to the CFTR C-terminus. The specific and tight association suggests a potential regulatory role of NHE-RF in cystic fibrosis transmembrane conductance regulator (CFTR) function.z 1998 Federation of European Biochemical Societies.

PDZ domains are multifunctional protein-interaction motifs that often bind to the C-terminus of protein targets. Nitric oxide (NO), an endogenous signaling molecule, plays critical roles in nervous, immune, and cardiovascular function. Although there are numerous physiological functions for neuron-derived NO, produced primarily by the neuronal NO synthase (nNOS), excess nNOS activity mediates brain injury in cerebral ischemia and in animal models of Parkinson's disease. Subcellular localization of nNOS activity must therefore be tightly regulated. To determine ligands for the PDZ domain of nNOS, we screened 13 billion distinct peptides and found that the nNOS-PDZ domain binds tightly to peptides ending Asp-X-Val. This differs from the only known (Thr/Ser)-X-Val consensus that interacts with PDZ domains from PSD-95. Preference for Asp at the -2 peptide position is mediated by Tyr-77 of nNOS. A Y77D78 to H77E78 substitution changes the binding specificity from Asp-X-Val to Thr-X-Val. Guided by the Asp-X-Val consensus, candidate nNOS interacting proteins have been identified including glutamate and melatonin receptors. Our results demonstrate that PDZ domains have distinct peptide binding specificity.

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Renewal of nongermative epithelia is poorly understood. The novel mitogen “lacritin” is apically secreted by several nongermative epithelia. We tested 17 different cell types and discovered that lacritin is preferentially mitogenic or prosecretory for those types that normally contact lacritin during its glandular outward flow. Mitogenesis is dependent on lacritin's C-terminal domain, which can form an α-helix with a hydrophobic face, as per VEGF's and PTHLP's respective dimerization or receptor-binding domain. Lacritin targets downstream NFATC1 and mTOR. The use of inhibitors or siRNA suggests that lacritin mitogenic signaling involves Gαi or Gαo–PKCα-PLC–Ca2+–calcineurin–NFATC1 and Gαi or Gαo–PKCα-PLC–phospholipase D (PLD)–mTOR in a bell-shaped, dose-dependent manner requiring the Ca2+ sensor STIM1, but not TRPC1. This pathway suggests the placement of transiently dephosphorylated and perinuclear Golgi–translocated PKCα upstream of both Ca2+ mobilization and PLD activation in a complex with PLCγ2. Outward flow of lacritin from secretory cells through ducts may generate a proliferative/secretory field as a different unit of cellular renewal in nongermative epithelia where luminal structures predominate.

Background: Homeostatic regulation of epithelia influences disease acquisition and aging. Results: Prosecretory mitogen lacritin stimulates FOXO3-ATG101 and FOXO1-ATG7 autophagic coupling and restores metabolic homeostasis. Conclusion: Lacritin is a homeostatic regulator. Significance: Exogenous lacritin restores prohomeostatic activity to tears from dry eye individuals.

Drosophila INAD, which contains five tandem protein interaction PDZ domains, plays an important role in the G protein-coupled visual signal transduction. Mutations in InaD alleles display mislocalization of signaling molecules of phototransduction which include the essential effector, phospholipase C-β (PLC-β), which is also known as NORPA. The molecular and biochemical details of this functional link are unknown. We report that INAD directly binds to NORPA via two terminally positioned PDZ1 and PDZ5 domains. PDZ1 binds to the C-terminus of NORPA, while PDZ5 binds to an internal region overlapping with the G box-homology region (a putative G protein-interacting site). The NORPA proteins lacking binding sites, which display normal basal PLC activity, can no longer associate with INAD in vivo. These truncations cause significant reduction of NORPA protein expression in rhabdomeres and severe defects in phototransduction. Thus, the two terminal PDZ domains of INAD, through intermolecular and/or intramolecular interactions, are brought into proximity in vivo. Such domain organization allows for the multivalent INAD-NORPA interactions which are essential for G protein-coupled phototransduction.

The putative glycine dehydrogenase of Mycobacterium tuberculosis catalyzes the reductive amination of glyoxylate to glycine but not the reverse reaction. The enzyme was purified and identified as the previously characterized alanine dehydrogenase. The Ald enzyme was expressed in Escherichia coli and had both pyruvate and glyoxylate aminating activities. The gene, ald, was inactivated in M. tuberculosis, which resulted in the loss of all activities. Both enzyme activities were found associated with the cell and were not detected in the extracellular filtrate. By using an anti-Ald antibody, the protein was localized to the cell membrane, with a smaller fraction in the cytosol. None was detected in the extracellular medium. The ald knockout strain grew without alanine or glycine and was able to utilize glycine but not alanine as a nitrogen source. Transcription of ald was induced when alanine was the sole nitrogen source, and higher levels of Ald enzyme were measured. Ald is proposed to have several functions, including ammonium incorporation and alanine breakdown. Mycobacterium tuberculosis is the causative agent of tuberculosis, and one of the most successful human pathogens. It was responsible for approximately 2 million deaths in 2008, while currently almost one-third of the world's population is infected with this organism. Research with M. tuberculosis has described a pathogen uniquely adapted to the wide range of harsh environments presented by the host. Much of this work has focused on the microbe's metabolism, with the idea of identification of novel enzymes or pathways to target for drug development.One of these environmental factors is nitrogen availability. Very little is known about the nitrogen sources used by M. tuberculosis in vivo. M. tuberculosis can utilize many amino acids for nitrogen, including alanine and glycine (29). Mutants of M. tuberculosis unable to synthesize proline retained the ability to replicate in the human macrophage cell line THP-1 (35), while other amino acid auxotroph mutants were attenuated (3,17,22,52). A Mycobacterium bovis BCG mutant unable to make methionine showed survival in mice similar to the wild-type strain (32). This suggests some amino acids are available in vivo and serve as nutrients for M. tuberculosis.The enzyme glycine dehydrogenase was first described in M. tuberculosis in 1962 (16). This enzyme was detected by the reductive amination of glyoxylate to glycine concurrent with the oxidation of NADH to NAD ϩ (Fig. 1). This reaction represents glyoxylate reductive aminase (GxRA) activity. The activity corresponding to the reverse reaction, catalyzed by glycine dehydrogenase (GDH), was not detected. The expression of glyoxylate reductive amination by a putative glycine dehydrogenase in M. tuberculosis has been characterized in nonreplicating persistent (NRP) cultures (58). In the Wayne model of dormancy, sealed cultures of M. tuberculosis create a microaerobic environment (NRP-1), which subsequently develops into the anaerobic stage (NRP-2) (58). GxRA activity w...

Cell surface heparan sulfate (HS) proteoglycans are carbohydrate-rich regulators of cell migratory, mitogenic, secretory, and inflammatory activity that bind and present soluble heparin-binding growth factors (e.g., fibroblast growth factor, Wnt, Hh, transforming growth factor β, amphiregulin, and hepatocyte growth factor) to their respective signaling receptors. We demonstrate that the deglycanated core protein of syndecan-1 (SDC1) and not HS chains nor SDC2 or -4, appears to target the epithelial selective prosecretory mitogen lacritin. An important and novel step in this mechanism is that binding necessitates prior partial or complete removal of HS chains by endogenous heparanase. This limits lacritin activity to sites where heparanase appears to predominate, such as sites of exocrine cell migration, secretion, renewal, and inflammation. Binding is mutually specified by lacritin's C-terminal mitogenic domain and SDC1's N terminus. Heparanase modification of the latter transforms a widely expressed HS proteoglycan into a highly selective surface-binding protein. This novel example of cell specification through extracellular modification of an HS proteoglycan has broad implications in development, homeostasis, and disease.