SH3 binding sites of ZG29p mediate an interaction with amylase and are involved in condensation-sorting in the exocrine rat pancreas

Proteomic Analysis of the β-Arrestin Interactomes

Protein-protein interaction is crucial to protein function and cellular signaling. A number of approaches were developed and applied to characterize protein-protein interactions in the past decades. In recent years, mass spectrometry (MS)-based proteomic methods have emerged as powerful tools to identify protein binding partners in a global and high-throughput manner. In this chapter, we describe the proteomic methods used to characterize the whole sets of proteins associated with β-arrestins (β-arrestin interactomes). The method starts with co-immunoprecipitation (co-IP) of β-arrestin signaling complexes from cells followed by protease digestion and LC/MS/MS analysis (liquid chromatography/tandem mass spectrometry) of proteins in the β-arrestin signaling complexes. To investigate changes in the amounts of binding partners under different conditions, we also describe a SILAC (stable isotope labeling by amino acids in cell culture) method to obtain quantitative information for β-arrestin interactomes.

Structure and function insights into the NuRD chromatin remodeling complex

Transcription regulation through chromatin compaction and decompaction is regulated through various chromatin-remodeling complexes such as nucleosome remodeling and histone deacetylation (NuRD) complex. NuRD is a 1 MDa multi-subunit protein complex which comprises many different subunits, among which histone deacetylases HDAC1/2, ATP-dependent remodeling enzymes CHD3/4, histone chaperones RbAp46/48, CpG-binding proteins MBD2/3, the GATAD2a (p66α) and/or GATAD2b (p66β) and specific DNA-binding proteins MTA1/2/3. Here, we review the currently known crystal and NMR structures of these subunits, the functional data and their relevance for biomedical research considering the implication of NuRD subunits in cancer and various other diseases. The complexity of this macromolecular assembly, and its poorly understood mode of interaction with the nucleosome, the repeating unit of chromatin, illustrate that this complex is a major challenge for structure-function relationship studies which will be tackled best by an integrated biology approach.

Molecular functions and significance of the MTA family in hormone-independent cancer

The members of the metastasis-associated protein (MTA) family play pivotal roles in both physiological and pathophysiological processes, especially in cancer development and metastasis, and their role as master regulators has come to light. Due to the fact that they were first identified as crucial factors in estrogen receptor-mediated breast cancer metastasis, most of the early studies focused on their hormone-dependent functions. However, the accumulating evidence shows that the members of MTA family are deregulated in most, if not all, the cancers studied so far. Therefore, the levels as well as the activities of the MTA family members are widely accepted as potential biomarkers for diagnosis, prognosis, and predictors of overall survival. They function differently in different cancers with specific mechanisms. p53 and HIF-1α appear to be the respectively common upstream and downstream regulator of the MTA family in both development and metastasis of a wide spectrum of cancers. Here, we review the expression and clinical significance of the MTA family, focusing on hormone-independent cancers. To illustrate the molecular mechanisms, we analyze the MTA family-related signaling pathways in different cancers. Finally, targeting the MTA family directly or the pathways involved in the MTA family indirectly could be invaluable strategies in the development of cancer therapeutics.

The Expanding Mi-2/NuRD Complexes: A Schematic Glance

This mini-review will schematically update the progress of the expanding Mi-2/Nucleosome Remodeling Deacetylase (NuRD) complexes in cancer and in normal development such as stemness, with a focus on mammals and the increasingly popular and powerful model organism Caenorhabditis elegans. The Mi-2/NuRD complexes control gene activity during the development of complex organisms. Every Mi-2/NuRD complex contains many different core polypeptides, which form distinct multifunctional complexes with specific context-dependent regulators. The Mi-2/NuRD complexes have unique ATP-dependent chromatin remodeling, histone deacetylase, demethylase activities and higher order chromatin organization. They can regulate the accessibility of transcription factors or repair proteins to DNA. In this review, we summarize our current knowleges in the composition, interaction and function of the subunits within the Mi-2/NuRD complex, the methodology used for the identification of Mi-2/NuRD complexes, as well as the clinical and therapeutic implications targeting the Mi-2/NuRD subunits.

Radioiodination of humic substances

The known IODO-GEN™-method [1] was adapted for radiolabeling of humic and fulvic acids with131I. The water insoluble oxidizing agent 1,3,4,6tetrachloro-3α,6α-diphenylglycoluril (IODO-GEN™) forms an iodous ion species (I+), which undergoes an electrophilic I/H-substitution on aromatic moieties of the humic and fulvic acids. This method offers mild conditions with a lesser extent of oxidative alterations of the target molecule, accompanied by an easy handling due to the virtual water-insolubility of the oxidizing agent. The method was optimized and different techniques were tested for the purification of the radioiodinated humic material. The yield of the labeling procedure varies between 45 and 75% depending on the provenance of the humic material and the applied purification method. A specific activity up to 40 MBq/mg was achieved. Furthermore, the known inherent photo-susceptibility of the iodinated humic substance and the influence of reducing agents were verified. An additional release of131I up to 20% and up to 35%, respectively were observed.

Amyloid precursor protein and amyloid beta-peptide bind to ATP synthase and regulate its activity at the surface of neural cells

Amyloid precursor protein (APP) and amyloid beta-peptide (Abeta) have been implicated in a variety of physiological and pathological processes underlying nervous system functions. APP shares many features with adhesion molecules in that it is involved in neurite outgrowth, neuronal survival and synaptic plasticity. It is, thus, of interest to identify binding partners of APP that influence its functions. Using biochemical cross-linking techniques we have identified ATP synthase subunit alpha as a binding partner of the extracellular domain of APP and Abeta. APP and ATP synthase colocalize at the cell surface of cultured hippocampal neurons and astrocytes. ATP synthase subunit alpha reaches the cell surface via the secretory pathway and is N-glycosylated during this process. Transfection of APP-deficient neuroblastoma cells with APP results in increased surface localization of ATP synthase subunit alpha. The extracellular domain of APP and Abeta partially inhibit the extracellular generation of ATP by the ATP synthase complex. Interestingly, the binding sequence of APP and Abeta is similar in structure to the ATP synthase-binding sequence of the inhibitor of F1 (IF(1)), a naturally occurring inhibitor of the ATP synthase complex in mitochondria. In hippocampal slices, Abeta and IF(1) similarly impair both short- and long-term potentiation via a mechanism that could be suppressed by blockade of GABAergic transmission. These observations indicate that APP and Abeta regulate extracellular ATP levels in the brain, thus suggesting a novel mechanism in Abeta-mediated Alzheimer's disease pathology.

MTA family of coregulators in nuclear receptor biology and pathology

Nuclear receptors (NRs) rely on coregulators (coactivators and corepressors) to modulate the transcription of target genes. By interacting with nucleosome remodeling complexes, NR coactivators potentiate transcription, whereas corepressors inhibit transcription of the target genes. Metastasis-associated proteins (MTA) represent an emerging family of novel NR coregulators. In general, MTA family members form independent nucleosome remodeling and deacetylation (NuRD) complexes and repress the transcription of different genes by recruiting histone deacetylases onto their target genes. However, MTA1 also acts as a coactivator in a promoter-context dependent manner. Recent findings that repression of estrogen receptor transactivation functions by MTA1, MTA1s, and MTA2 and regulation of MTA3 by estrogen signaling have indicated the significance of these proteins in NR signaling. Here, we highlight the action of MTA proteins on NR signaling and their roles in pathophysiological conditions.

Binding of the Golgi sorting receptor muclin to pancreatic zymogens through sulfated O-linked oligosaccharides

Sorting and packaging of regulated secretory proteins involves protein aggregation in the trans-Golgi network and secretory granules. In this work, we characterized the pH-dependent interactions of pancreatic acinar cell-regulated secretory proteins (zymogens) with Muclin, a putative Golgi cargo receptor. In solution, purified Muclin co-aggregated with isolated zymogens at mildly acidic pH. In an overlay assay, [35S]sulfate biosynthetically labeled Muclin bound directly at mildly acidic pH to the zymogen granule content proteins amylase, prolipase, pro-carboxypeptidase A1, pro-elastase II, chymotrypsinogen B, and Reg1. Denaturation of Muclin with reducing agents to break the numerous intrachain disulfide bonds in Muclin's scavenger receptor cysteine-rich and CUB domains did not interfere with binding. Non-sulfated [35S]Met/Cys-labeled Muclin showed decreased binding in the overlay assay. Extensive Pronase E digestion of unlabeled Muclin was used to produce glycopeptides, which competed for binding of [35S]sulfate-labeled Muclin to zymogens. The results demonstrate that the sulfated, O-glycosylated groups are responsible for the pH-dependent interactions of Muclin with the zymogens. The behavior of Muclin fulfils the requirement of a Golgi cargo receptor to bind to regulated secretory proteins under the mildly acidic pH conditions that exist in the trans-Golgi network.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

Localization of VIP36 in the post-Golgi secretory pathway also of rat parotid acinar cells

VIP36 (36-kD vesicular integral membrane protein), originally purified from Madin-Darby canine kidney (MDCK) epithelial cells, belongs to a family of animal lectins and may act as a cargo receptor. To understand its role in secretory processes, we performed morphological analysis of the rat parotid gland. Immunoelectron microscopy provided evidence that endogenous VIP36 is localized in the trans-Golgi network, on immature granules, and on mature secretory granules in acinar cells. Double-staining immunofluorescence experiments confirmed that VIP36 and amylase co-localized in the apical regions of the acinar cells. This is the first study to demonstrate that endogenous VIP36 is involved in the post-Golgi secretory pathway, suggesting that VIP36 plays a role in trafficking and sorting of secretory and/or membrane proteins during granule formation.

Role of sulfated O-linked glycoproteins in zymogen granule formation

Packaging of proteins into regulated secretory granules is mediated by the mildly acidic pH of the trans Golgi network and immature secretory granules. This need for an acidic pH indicates that ionic interactions are important. The mouse pancreatic acinar cell contains four major sulfated glycoproteins,including the zymogen granule structural component Muclin. I tested the hypothesis that sulfation and the O-linked glycosylation to which the sulfates are attached are required for normal formation of zymogen granules in the exocrine pancreas. Post-translational processing was perturbed with two chemicals: sodium chlorate was used to inhibit sulfation and benzyl-N-acetyl-α-galactosaminide was used to inhibit O-linked oligosaccharide elongation. Both chemicals resulted in the accumulation in the Golgi region of the cell of large vacuoles that appear to be immature secretory granules, and the effect was much more extensive with benzyl-N-acetyl-α-galactosaminide than chlorate. Both chemical treatments inhibited basal secretion at prolonged chase times, and again benzyl-N-acetyl-α-galactosaminide had a greater effect than chlorate. In addition, benzyl-N-acetyl-α-galactosaminide, but not chlorate, totally inhibited stimulated secretion of newly synthesized proteins. These data provide evidence for a role of sulfated O-linked glycoproteins in protein condensation and maturation of zymogen granules. Under maximal inhibition of O-linked oligosaccharide biosynthesis, anterograde post-Golgi traffic in the regulated pathway is almost totally shut down, demonstrating the importance of these post-translational modifications in progression of secretory proteins through the regulated pathway and normal granule formation in the pancreatic acinar cell.

Interaction of syncollin with GP-2, the major membrane protein of pancreatic zymogen granules, and association with lipid microdomains

Syncollin, a novel pancreatic zymogen granule protein, is present on the luminal side of the granule membrane. To address the function of syncollin, we searched for putative binding partners. Cross-linking experiments with purified syncollin, and granule content and membrane proteins revealed a direct interaction between syncollin and GP-2, a major glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein. An interaction was also observed when cross-linking was performed with recombinant GP-2. In addition, syncollin could be cross-linked to itself, supporting the suggestion that it exists as a homo-oligomer. Cleavage of the GPI anchor of GP-2 by treatment of granule membranes with phosphatidylinositol-specific phospholipase C had no effect on the membrane attachment of syncollin, indicating that it is not mediated exclusively via an interaction with GP-2. Syncollin was found to be associated with detergent-insoluble cholesterol/glycolipid-enriched complexes. These complexes floated to the lighter fractions of sucrose-density gradients and also contained GP-2, the lectin ZG16p, sulphated matrix proteoglycans and the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) syntaxin 3 and synaptobrevin 2. Our results indicate that membrane-associated syncollin is a component of lipid rafts, where it interacts both with GP-2 and membrane lipids. We suggest that the syncollin-GP-2 complex might play a role in signal transduction across the granule membrane.