Amino acid residues critical for endoplasmic reticulum export and trafficking of platelet-activating factor receptor

Constitutively active GPR43 is crucial for proper leukocyte differentiation

The G protein-coupled receptors, GPR43 (free fatty acid receptor 2, FFA2) and GPR41 (free fatty acid receptor 3, FFA3), are activated by short-chain fatty acids produced under various conditions, including microbial fermentation of carbohydrates. Previous studies have implicated this receptor energy homeostasis and immune responses as well as in cell growth arrest and apoptosis. Here, we observed the expression of both receptors in human blood cells and a remarkable enhancement in leukemia cell lines (HL-60, U937, and THP-1 cells) during differentiation. A reporter assay revealed that GPR43 is coupled with Gα_i and Gα_12/13 and is constitutively active without any stimuli. Specific blockers of GPR43, GLPG0974 and CATPB function as inverse agonists because treatment with these compounds significantly reduces constitutive activity. In HL-60 cells, enhanced expression of GPR43 led to growth arrest through Gα_12/13 . In addition, the blockage of GPR43 activity in these cells significantly impaired their adherent properties due to the reduction of adhesion molecules. We further revealed that enhanced GPR43 activity induces F-actin formation. However, the activity of GPR43 did not contribute to butyrate-induced apoptosis in differentiated HL-60 cells because of the ineffectiveness of the inverse agonist on cell death. Collectively, these results suggest that GPR43, which possesses constitutive activity, is crucial for growth arrest, followed by the proper differentiation of leukocytes.

A Naturally Occurring Splice Variant of GGA1 Inhibits the Anterograde Post-Golgi Traffic of α2B-Adrenergic Receptor

The regulatory mechanisms of cell surface targeting of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum through the Golgi remain poorly understood. We have recently demonstrated that three Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding proteins (GGAs) mediate the post-Golgi export of α_2B-adrenergic receptor (α_2B-AR), a prototypic GPCR, and directly interact with the receptor. In particular, GGA1 interaction with α_2B-AR is mediated via its hinge domain. Here we determined the role of a naturally occurring truncated form of GGA1 (GGA1t) which lacks the N-terminal portion of the hinge domain in α_2B-AR trafficking and elucidated the underlying mechanisms. We demonstrated that both GGA1 and GGA1t were colocalized and mainly expressed at the Golgi. In marked contrast to GGA1, the expression of GGA1t significantly attenuated the cell surface export of newly synthesized α_2B-AR from the Golgi and in parallel receptor-mediated signaling. Furthermore, we found that GGA1t formed homodimers and heterodimers with GGA1. More interestingly, GGA1t was unable to bind the cargo α_2B-AR and to recruit clathrin onto the trans-Golgi network. These data provide evidence implicating that the truncated form of GGA1 behaviors as a dominant-negative regulator for the cell surface export of α_2B-AR and this function of GGA1t is attributed to its abilities to dimerize with its wide type counterpart and to inhibit cargo interaction and clathrin recruitment to form specialized transport vesicles.

Staphylococcus aureus toxin LukSF dissociates from its membrane receptor target to enable renewed ligand sequestration

Staphylococcus aureus Panton-Valentine leukocidin is a pore-forming toxin targeting the human C5a receptor (hC5aR), enabling this pathogen to battle the immune response by destroying phagocytes through targeted lysis. The mechanisms that contribute to rapid cell lysis are largely unexplored. Here, we show that cell lysis may be enabled by a process of toxins targeting receptor clusters and present indirect evidence for receptor "recycling" that allows multiple toxin pores to be formed close together. With the use of live cell single-molecule super-resolution imaging, Förster resonance energy transfer and nanoscale total internal reflection fluorescence colocalization microscopy, we visualized toxin pore formation in the presence of its natural docking ligand. We demonstrate disassociation of hC5aR from toxin complexes and simultaneous binding of new ligands. This effect may free mobile receptors to amplify hyperinflammatory reactions in early stages of microbial infections and have implications for several other similar bicomponent toxins and the design of new antibiotics.-Haapasalo, K., Wollman, A. J. M., de Haas, C. J. C., van Kessel, K. P. M., van Strijp, J. A. G., Leake, M. C. Staphylococcus aureus toxin LukSF dissociates from its membrane receptor target to enable renewed ligand sequestration.

Endosomal vacuoles of the prepupal salivary glands of Drosophila play an essential role in the metabolic reallocation of iron

In the recent past, we demonstrated that a great deal is going on in the salivary glands of Drosophila in the interval after they release their glycoprotein-rich secretory glue during pupariation. The early-to-mid prepupal salivary glands undergo extensive endocytosis with widespread vacuolation of the cytoplasm followed by massive apocrine secretion. Here, we describe additional novel properties of these endosomes. The use of vital pH-sensitive probes provided confirmatory evidence that these endosomes have acidic contents and that there are two types of endocytosis seen in the prepupal glands. The salivary glands simultaneously generate mildly acidic, small, basally-derived endosomes and strongly acidic, large and apical endosomes. Staining of the large vacuoles with vital acidic probes is possible only after there is ambipolar fusion of both basal and apical endosomes, since only basally-derived endosomes can bring fluorescent probes into the vesicular system. We obtained multiple lines of evidence that the small basally-derived endosomes are chiefly involved in the uptake of dietary Fe³⁺ iron. The fusion of basal endosomes with the larger and strongly acidic apical endosomes appears to facilitate optimal conditions for ferrireductase activity inside the vacuoles to release metabolic Fe²⁺ iron. While iron was not detectable directly due to limited staining sensitivity, we found increasing fluorescence of the glutathione-sensitive probe CellTracker Blue CMAC in large vacuoles, which appeared to depend on the amount of iron released by ferrireductase. Moreover, heterologous fluorescently-labeled mammalian iron-bound transferrin is actively taken up, providing direct evidence for active iron uptake by basal endocytosis. In addition, we serendipitously found that small (basal) endosomes were uniquely recognized by PNA lectin, whereas large (apical) vacuoles bound DBA lectin.

Biomolecular engineering for nanobio/bionanotechnology

Biomolecular engineering can be used to purposefully manipulate biomolecules, such as peptides, proteins, nucleic acids and lipids, within the framework of the relations among their structures, functions and properties, as well as their applicability to such areas as developing novel biomaterials, biosensing, bioimaging, and clinical diagnostics and therapeutics. Nanotechnology can also be used to design and tune the sizes, shapes, properties and functionality of nanomaterials. As such, there are considerable overlaps between nanotechnology and biomolecular engineering, in that both are concerned with the structure and behavior of materials on the nanometer scale or smaller. Therefore, in combination with nanotechnology, biomolecular engineering is expected to open up new fields of nanobio/bionanotechnology and to contribute to the development of novel nanobiomaterials, nanobiodevices and nanobiosystems. This review highlights recent studies using engineered biological molecules (e.g., oligonucleotides, peptides, proteins, enzymes, polysaccharides, lipids, biological cofactors and ligands) combined with functional nanomaterials in nanobio/bionanotechnology applications, including therapeutics, diagnostics, biosensing, bioanalysis and biocatalysts. Furthermore, this review focuses on five areas of recent advances in biomolecular engineering: (a) nucleic acid engineering, (b) gene engineering, (c) protein engineering, (d) chemical and enzymatic conjugation technologies, and (e) linker engineering. Precisely engineered nanobiomaterials, nanobiodevices and nanobiosystems are anticipated to emerge as next-generation platforms for bioelectronics, biosensors, biocatalysts, molecular imaging modalities, biological actuators, and biomedical applications.

A clickable neurosteroid photolabel reveals selective Golgi compartmentalization with preferential impact on proximal inhibition

Anesthetic, GABA-active neurosteroids potently augment GABAA receptor function, leading to important behavioral consequences. Neurosteroids and their synthetic analogues are also models for a wide variety of cell-permeant neuroactive compounds. Cell permeation and compartmentalization raise the possibility that these compounds' actions are influenced by their cellular partitioning, but these contributions are not typically considered experimentally or therapeutically. To examine the interplay between cellular accumulation and pharmacodynamics of neurosteroids, we synthesized a novel chemical biology analogue (bio-active, clickable photolabel) of GABA-active neurosteroids. We discovered that the analogue selectively photo-labels neuronal Golgi in rat hippocampal neurons. The active analogue's selective distribution was distinct from endogenous cholesterol and not completely shared by some non-GABA active, neurosteroid-like analogues. On the other hand, the distribution was not enantioselective and did not require energy, in contrast to other recent precedents from the literature. We demonstrate that the soma-selective accumulation can act as a sink or source for steroid actions at plasma-membrane GABA receptors, altering steady-state and time course of effects at somatic GABAA receptors relative to dendritic receptors. Our results suggest a novel mechanism for compartment-selective drug actions at plasma-membrane receptors.

The atypical N-glycosylation motif, Asn-Cys-Cys, in human GPR109A is required for normal cell surface expression and intracellular signaling

Asparagine-linked glycosylation (N-glycosylation) is necessary for the proper folding of secreted and membrane proteins, including GPCRs. Thus, many GPCRs possess the N-glycosylation motif Asn-X-Ser/Thr at their N-termini and/or extracellular loops. We found that human GPR109A (hGPR109A) has an N-glycosylation site at Asn(17) in the N-terminal atypical motif, Asn(17)-Cys(18)-Cys(19). Why does hGPR109A require the atypical motif, rather than the typical sequence? Here we show that Asn(17)-Cys(18)-Cys(19) sequence of hGPR109A possesses 2 biologic roles. First, Asn(17)-X-Cys(19) contributed to hGPR109A N-glycosylation by acting as an atypical motif. This modification is required for the normal surface expression of hGPR109A, as evidenced by the reduced surface expression of the nonglycosylated mutants, hGPR109A/N17A, and the finding that hGPR109A/C19S and hGPR109A/C19T, which are N-glycosylated at Asn(17), exhibited expression similar to the wild-type receptor. Second, the X-Cys(18)-Cys(19) dicysteine is indispensable for hGPR109A function. Substitution of Cys(18) or Cys(19) residue to Ala impaired Gi-mediated signaling via hGPR109A. We propose the disulfide bond formations of these residues with other Cys existed in the extracellular loops for the proper folding. Together, these results suggest that the atypical motif Asn(17)-Cys(18)-Cys(19) is crucial for the normal surface trafficking and function of hGPR109A.

Regulation of α2B-Adrenerigc Receptor Export Trafficking by Specific Motifs

Intracellular trafficking and precise targeting to specific locations of G protein-coupled receptors (GPCRs) control the physiological functions of the receptors. Compared to the extensive efforts dedicated to understanding the events involved in the endocytic and recycling pathways, the molecular mechanisms underlying the transport of the GPCR superfamily from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane are relatively less well defined. Over the past years, we have used α(2B)-adrenergic receptor (α(2B)-AR) as a model to define the factors that control GPCR export trafficking. In this chapter, we will review specific motifs identified to mediate the export of nascent α(2B)-AR from the ER and the Golgi and discuss the possible underlying mechanisms. As these motifs are highly conserved among GPCRs, they may provide common mechanisms for export trafficking of these receptors.

Visualization of the pH-dependent dynamic distribution of G2A in living cells

G2A (from G2 accumulation) receptor is a member of the proton-sensing G-protein coupled receptor (GPCR) family and induces signal transduction events that regulate the cell cycle, proliferation, oncogenesis, and immunity. The mechanism by which G2A-mediated signal transduction is regulated by the extracellular pH remains unresolved. Here, we first visualize the pH-dependent G2A distribution change in living cells by a sortase A-mediated pulse labeling technology: the short-peptide tag-fused human G2A on human embryo kidney HEK293T cell surfaces was labeled with a small fluorescent dye in the presence of lysophosphatidylcholine, and the labeled G2A was chased at acidic and neutral pHs in real time by microscope time course observations. G2A internalization from cell surfaces into intracellular compartments was observed to be inhibited under acidic pH conditions, and this inhibition was relieved at neutral pH. Additionally, the internalized G2A was redistributed onto cell surfaces by jumping from a neutral to an acidic pH. From quantitative image analysis data, we conclude the amount of G2A on the cell surface was controlled by suppressing the G2A internalization rate by one-tenth in response to the extracellular acidic pH, and this acidic pH-induced G2A accumulation on cell surfaces may be explained by proton-induced dissociation of G2A from endocytic machinery.

Anchoring foreign substances on live cell surfaces using Sortase A specific binding peptide

A simple, rapid, and efficient live cell surface labeling method has been developed that uses a direct conjugation between Sortase A expressed transiently at the cell surface and Sortase A specific binding peptide.

A single lys residue on the first intracellular loop modulates the endoplasmic reticulum export and cell-surface expression of α2A-adrenergic receptor

Export from the endoplasmic reticulum (ER) represents an initial step in intracellular trafficking of G protein-coupled receptors (GPCRs). However, the underlying molecular mechanisms remain poorly understood. We have previously demonstrated that a highly conserved Leu residue on the first intracellular loop (ICL1) is required for exit of several GPCRs from the ER. Here we found that, in addition to Leu64 residue in the ICL1, the neighboring positively charged residue Lys65also modulates the cell-surface transport of α_2A-adrenergic receptor (α_2A-AR). Mutation of Lys65 to Ala, Glu and Gln significantly attenuated, whereas mutation of Lys65 to Arg strongly augmented α_2A-AR expression at the cell surface. Consistent with the effects on the cell-surface expression of α_2A-AR, mutation of Lys65 to Ala and Arg produced opposing effects on α_2A-AR-mediated ERK1/2 activation. Furthermore, confocal microscopy revealed that the α_2A-AR mutant K65A displayed a strong intracellular expression pattern and was extensively co-localized with the ER marker DsRed2-ER, suggestive of ER accumulation. These data provide the first evidence indicating an important function for a single Lys residue on the ICL1 in the ER export and cell-surface expression of α_2A-AR. These data also suggest that the ICL1 may possess multiple signals that control the cell-surface targeting of GPCRs via distinct mechanisms.

Design of Ca2+-independent Staphylococcus aureus sortase A mutants

The catalytic activity of Staphylococcus aureus sortase A (SaSrtA) is dependent on Ca(2+), because binding of Ca(2+) to Glu residues distal to the active site stabilizes the substrate binding site. To obtain Ca(2+)-independent SaSrtA, we substituted two Glu residues in the Ca(2+)-binding pocket (Glu(105) and Glu(108)). Although single mutations decreased SaSrtA activity, mutations of both Glu(105) and Glu(108) resulted in Ca(2+)-independent activity. Kinetic analysis suggested that the double mutations affect the substrate binding site, without affecting substrate specificity. This approach will allow us to develop SaSrtA variants suitable for various applications, including in vivo site-specific protein modification and labeling.

Protein ligation in living cells using sortase

Sortagging is a versatile method for site-specific modification of proteins as applied to a variety of in vitro reactions. Here, we explore possibilities of adapting the sortase method for use in living cells. For intracellular sortagging, we employ the Ca²⁺-independent sortase A transpeptidase (SrtA) from Streptococcus pyogenes. Substrate proteins were equipped with the C-terminal sortase-recognition motif (LPXTG); we used proteins with an N-terminal (oligo)glycine as nucleophiles. We show that sortase-dependent protein ligation can be achieved in Saccharomyces cerevisiae and in mammalian HEK293T cells, both in the cytosol and in the lumen of the endoplasmic reticulum (ER). ER luminal sortagging enables secretion of the reaction products, among which circular polypeptides. Protein ligation of substrate and nucleophile occurs within 30 min of translation. The versatility of the method is shown by protein ligation of multiple substrates with green fluorescent protein-based nucleophiles in different intracellular compartments.

Making and breaking peptide bonds: protein engineering using sortase

Sortases are a class of bacterial enzymes that possess transpeptidase activity. It is their ability to site-specifically break a peptide bond and then reform a new bond with an incoming nucleophile that makes sortase an attractive tool for protein engineering. This technique has been adopted for a range of applications, from chemistry-based to cell biology and technology. In this Minireview we provide a brief overview of the biology of sortase enzymes and current applications in protein engineering. We identify areas that lend themselves to further innovation and that suggest new applications.

Enhancement of sortase A-mediated protein ligation by inducing a β-hairpin structure around the ligation site

A Staphylococcus aureus transpeptidase, sortase A (SrtA), catalyzes selective peptide/protein ligations that have been applied to cell imaging and protein engineering, while the ligations do not proceed to completion due to their reversibility. We successfully enhanced SrtA-mediated protein ligation through the formation of a β-hairpin around the ligation site.

Specific ligands as pharmacological chaperones: The transport of misfolded G-protein coupled receptors to the cell surface

In the endoplasmic reticulum (ER), quality control mechanisms distinguish between correctly and incorrectly folded structures to ensure that aberrant proteins are not processed along the secretory pathway. Numerous studies have demonstrated the functional rescue of ER-retained, aberrant proteins by small membrane permeable molecules called pharmacological chaperones. Pharmacological chaperones can bind to misfolded proteins, including G-protein coupled receptors (GPCRs), and promote their correct folding and export from the ER. Recently, common structural features of GPCRs have been uncovered, including the eighth helical domain in the C-terminal tail and conserved residues in the transmembrane domains. However, little is known about the importance of these features in signaling and intracellular trafficking, because receptors deficient in these domains are likely retained in the ER due to misfolding. In this review, we summarize the current knowledge about the requirement of these consensus domains and amino acid residues for the passing through the quality control of the ER. Furthermore, we propose the utilization of membrane permeable ligands for the transport of their cognate, ER-retained GPCRs to the cell surface. The chaperone activity of these ligands allows us to perform functional analyses of the structure-deficient receptors after their trafficking to the cell surface.