Structural basis for the binding of tryptophan-based motifs by δ-COP.

Coatomer consists of two subcomplexes: the membrane-targeting, ADP ribosylation factor 1 (Arf1):GTP-binding ßγδζ-COP F-subcomplex, which is related to the adaptor protein (AP) clathrin adaptors, and the cargo-binding αß'ε-COP B-subcomplex. We present the structure of the C-terminal µ-homology domain of the yeast δ-COP subunit in complex with the WxW motif from its binding partner, the endoplasmic reticulum-localized Dsl1 tether. The motif binds at a site distinct from that used by the homologous AP µ subunits to bind YxxΦ cargo motifs with its two tryptophan residues sitting in compatible pockets. We also show that the Saccharomyces cerevisiae Arf GTPase-activating protein (GAP) homolog Gcs1p uses a related WxxF motif at its extreme C terminus to bind to δ-COP at the same site in the same way. Mutations designed on the basis of the structure in conjunction with isothermal titration calorimetry confirm the mode of binding and show that mammalian δ-COP binds related tryptophan-based motifs such as that from ArfGAP1 in a similar manner. We conclude that δ-COP subunits bind Wxn(1-6)[WF] motifs within unstructured regions of proteins that influence the lifecycle of COPI-coated vesicles; this conclusion is supported by the observation that, in the context of a sensitizing domain deletion in Dsl1p, mutating the tryptophan-based motif-binding site in yeast causes defects in both growth and carboxypeptidase Y trafficking/processing.

Drebrins and Connexins: A Biomedical Perspective.

In this chapter we summarize knowledge on the role of drebrin in cell-cell communications. Specifically, we follow drebrin-connexin-43 interactions and drebrin behavior at the cell-cell interface described earlier. Drebrin is a part of the actin cytoskeleton which is a target of numerous bacteria and viruses invading mammalian cells. Drebrin phosphorylation, self-inhibition and transition between filaments, particles, and podosomes underlie cellular mechanisms involved in diseases and cognitive disorders. Cytoskeletal rearrangements influence the state of gap junction contacts which regulate cell signaling and metabolic flow of information across cells in tissues. Taking into account that connexin-43 (Cx43) (together with Cx30) is heavily expressed in astrocytes and that drebrin supports cell-cell contacts, the understanding of details of how brain cells live and die reveals molecular pathology involved in neurodegeneration, Alzheimer's disease (AD), other cognitive disorders, and aging.Bidirectional connexin channels are permeable to Ca2+ ions, IP3, ATP, and cAMP. Connexin hemichannels are important for paracrine regulation and can release and exchange energy with other cells using ATP to transfer information and to support damaged cells. Connexin channels, hemichannels, and adhesion plaques are regulated by assembly and disassembly of the actin cytoskeleton. Drebrin degradation can alter gap junction communication, and drebrin level is decreased in the brain of AD patients. The diversity of drebrin functions in neurons, astrocytes, and non-neuronal cells still remains to be revealed. We believe that the knowledge on drebrin summarized here will contribute to key questions, "covering the gap" between cell-cell communications and the submembrane cytoskeleton.

Regulated oligomerization induces uptake of a membrane protein into COPII vesicles independent of its cytosolic tail.

Export of transmembrane proteins from the endoplasmic reticulum (ER) is driven by directed incorporation into coat protein complex II (COPII)-coated vesicles. The sorting of some cargo proteins into COPII vesicles was shown to be mediated by specific interactions between transmembrane and COPII-coat-forming proteins. But even though some signals for ER exit have been identified on the cytosolic domains of membrane proteins, the general signaling and sorting mechanisms of ER export are still poorly understood. To investigate the role of cargo protein oligomer formation in the export process, we have created a transmembrane fusion protein that - owing to its FK506-binding protein domains - can be oligomerized in isolated membranes by addition of a small-molecule dimerizer. Packaging of the fusion protein into COPII vesicles is strongly enhanced in the presence of the dimerizer, demonstrating that the oligomeric state is an ER export signal for this membrane protein. Surprisingly, the cytosolic tail is not required for this oligomerization-dependent effect on protein sorting. Thus, an alternative mechanism, such as membrane bending, must account for ER export of the fusion protein.

Fast structural responses of gap junction membrane domains to AB5 toxins.

Gap junctions (GJs) represent connexin-rich membrane domains that connect interiors of adjoining cells in mammalian tissues. How fast GJs can respond to bacterial pathogens has not been known previously. Using Bessel beam plane illumination and confocal spinning disk microscopy, we found fast (~500 ms) formation of connexin-depleted regions (CDRs) inside GJ plaques between cells exposed to AB5 toxins. CDR formation appears as a fast redistribution of connexin channels within GJ plaques with minor changes in outline or geometry. CDR formation does not depend on membrane trafficking or submembrane cytoskeleton and has no effect on GJ conductance. However, CDR responses depend on membrane lipids, can be modified by cholesterol-clustering agents and extracellular K(+) ion concentration, and influence cAMP signaling. The CDR response of GJ plaques to bacterial toxins is a phenomenon observed for all tested connexin isoforms. Through signaling, the CDR response may enable cells to sense exposure to AB5 toxins. CDR formation may reflect lipid-phase separation events in the biological membrane of the GJ plaque, leading to increased connexin packing and lipid reorganization. Our data demonstrate very fast dynamics (in the millisecond-to-second range) within GJ plaques, which previously were considered to be relatively stable, long-lived structures.

Functional characterization of bovine viral diarrhea virus nonstructural protein 5A by reverse genetic analysis and live cell imaging.

Nonstructural protein 5A (NS5A) of bovine viral diarrhea virus (BVDV) is a hydrophilic phosphoprotein with RNA binding activity and a critical component of the viral replicase. In silico analysis suggests that NS5A encompasses three domains interconnected by two low-complexity sequences (LCSs). While domain I harbors two functional determinants, an N-terminal amphipathic helix important for membrane association, and a Zn-binding site essential for RNA replication, the structure and function of the C-terminal half of NS5A are still ill defined. In this study, we introduced a panel of 10 amino acid deletions covering the C-terminal half of NS5A. In the context of a highly efficient monocistronic replicon, deletions in LCS I and the N-terminal part of domain II, as well as in domain III, were tolerated with regard to RNA replication. When introduced into a bicistronic replicon, only deletions in LCS I and the N-terminal part of domain II were tolerated. In the context of the viral full-length genome, these mutations allowed residual virion morphogenesis. Based on these data, a functional monocistronic BVDV replicon coding for an NS5A variant with an insertion of the fluorescent protein mCherry was constructed. Live cell imaging demonstrated that a fraction of NS5A-mCherry localizes to the surface of lipid droplets. Taken together, this study provides novel insights into the functions of BVDV NS5A. Moreover, we established the first pestiviral replicon expressing fluorescent NS5A-mCherry to directly visualize functional viral replication complexes by live cell imaging.

Molecular insights into vesicle tethering at the Golgi by the conserved oligomeric Golgi (COG) complex and the golgin TATA element modulatory factor (TMF).

Protein sorting between eukaryotic compartments requires vesicular transport, wherein tethering provides the first contact between vesicle and target membranes. Here we map and start to functionally analyze the interaction network of the conserved oligomeric Golgi (COG) complex that mediates retrograde tethering at the Golgi. The interactions of COG subunits with members of transport factor families assign the individual subunits as specific interaction hubs. Functional analysis of selected interactions suggests a mechanistic tethering model. We find that the COG complex interacts with two different Rabs in addition to each end of the golgin "TATA element modulatory factor" (TMF). This allows COG to potentially bridge the distance between the distal end of the golgin and the target membrane thereby promoting tighter docking. Concurrently we show that the central portion of TMF can bind to Golgi membranes that are liberated of their COPI cover. This latter interaction could serve to bring vesicle and target membranes into close apposition prior to fusion. A target selection mechanism, in which a hetero-oligomeric tethering factor organizes Rabs and coiled transport factors to enable protein sorting specificity, could be applicable to vesicle targeting throughout eukaryotic cells.

A streamlined approach to structure elucidation using in cellulo crystallized recombinant proteins, InCellCryst.

With the advent of serial X-ray crystallography on microfocus beamlines at free-electron laser and synchrotron facilities, the demand for protein microcrystals has significantly risen in recent years. However, by in vitro crystallization extensive efforts are usually required to purify proteins and produce sufficiently homogeneous microcrystals. Here, we present InCellCryst, an advanced pipeline for producing homogeneous microcrystals directly within living insect cells. Our baculovirus-based cloning system enables the production of crystals from completely native proteins as well as the screening of different cellular compartments to maximize chances for protein crystallization. By optimizing cloning procedures, recombinant virus production, crystallization and crystal detection, X-ray diffraction data can be collected 24 days after the start of target gene cloning. Furthermore, improved strategies for serial synchrotron diffraction data collection directly from crystals within living cells abolish the need to purify the recombinant protein or the associated microcrystals.

Inhibition of mTOR induces autophagy and reduces toxicity of polyglutamine expansions in fly and mouse models of Huntington disease.

Huntington disease is one of nine inherited neurodegenerative disorders caused by a polyglutamine tract expansion. Expanded polyglutamine proteins accumulate abnormally in intracellular aggregates. Here we show that mammalian target of rapamycin (mTOR) is sequestered in polyglutamine aggregates in cell models, transgenic mice and human brains. Sequestration of mTOR impairs its kinase activity and induces autophagy, a key clearance pathway for mutant huntingtin fragments. This protects against polyglutamine toxicity, as the specific mTOR inhibitor rapamycin attenuates huntingtin accumulation and cell death in cell models of Huntington disease, and inhibition of autophagy has the converse effects. Furthermore, rapamycin protects against neurodegeneration in a fly model of Huntington disease, and the rapamycin analog CCI-779 improved performance on four different behavioral tasks and decreased aggregate formation in a mouse model of Huntington disease. Our data provide proof-of-principle for the potential of inducing autophagy to treat Huntington disease.

ArfGAP1 dynamics and its role in COPI coat assembly on Golgi membranes of living cells.

Secretory protein trafficking relies on the COPI coat, which by assembling into a lattice on Golgi membranes concentrates cargo at specific sites and deforms the membranes at these sites into coated buds and carriers. The GTPase-activating protein (GAP) responsible for catalyzing Arf1 GTP hydrolysis is an important part of this system, but the mechanism whereby ArfGAP is recruited to the coat, its stability within the coat, and its role in maintenance of the coat are unclear. Here, we use FRAP to monitor the membrane turnover of GFP-tagged versions of ArfGAP1, Arf1, and coatomer in living cells. ArfGAP1 underwent fast cytosol/Golgi exchange with approximately 40% of the exchange dependent on engagement of ArfGAP1 with coatomer and Arf1, and affected by secretory cargo load. Permanent activation of Arf1 resulted in ArfGAP1 being trapped on the Golgi in a coatomer-dependent manner. These data suggest that ArfGAP1, coatomer and Arf1 play interdependent roles in the assembly-disassembly cycle of the COPI coat in vivo.

Limiting transport steps and novel interactions of Connexin-43 along the secretory pathway.

Connexins are four-transmembrane-domain proteins expressed in all vertebrates which form permeable gap junction channels that connect cells. Here, we analysed Connexin-43 (Cx43) transport to the plasma membrane and studied the effects of small GTPases acting along the secretory pathway. We show that both GTP- and GDP-restricted Sar1 prevents exit of Cx43 from the endoplasmic reticulum (ER), but only GTP-restricted Sar1 arrests Cx43 in COP II-coated ER exit sites and accumulates 14-3-3 proteins in the ER fraction. FRET-FLIM data confirm that already in ER exit sites Cx43 exists in oligomeric form, suggesting an in vivo role for 14-3-3 in Cx43 oligomerization. Exit of Cx43 from the ER can be blocked by other factors--such as expression of the beta subunit of the COP I coat or p50/dynamitin that acts on the microtubule-based dynein motor complex. GTP-restricted Arf1 blocks Cx43 in the Golgi. Lastly, we show that GTP-restricted Arf6 removes Cx43 gap junction plaques from the cell-cell interface and targets them to degradation. These data provide a molecular explanation of how small GTPases act to regulate Cx43 transport through the secretory pathway, facilitating or abolishing cell-cell communication through gap junctions.

The Sec34/Sec35p complex, a Ypt1p effector required for retrograde intra-Golgi trafficking, interacts with Golgi SNAREs and COPI vesicle coat proteins.

The Sec34/35 complex was identified as one of the evolutionarily conserved protein complexes that regulates a cis-Golgi step in intracellular vesicular transport. We have identified three new proteins that associate with Sec35p and Sec34p in yeast cytosol. Mutations in these Sec34/35 complex subunits result in defects in basic Golgi functions, including glycosylation of secretory proteins, protein sorting, and retention of Golgi resident proteins. Furthermore, the Sec34/35 complex interacts genetically and physically with the Rab protein Ypt1p, intra-Golgi SNARE molecules, as well as with Golgi vesicle coat complex COPI. We propose that the Sec34/35 protein complex acts as a tether that connects cis-Golgi membranes and COPI-coated, retrogradely targeted intra-Golgi vesicles.

ARF-GAP-mediated interaction between the ER-Golgi v-SNAREs and the COPI coat.

In eukaryotic cells, secretion is achieved by vesicular transport. Fusion of such vesicles with the correct target compartment relies on SNARE proteins on both vesicle (v-SNARE) and the target membranes (t-SNARE). At present it is not clear how v-SNAREs are incorporated into transport vesicles. Here, we show that binding of ADP-ribosylation factor (ARF)-GTPase-activating protein (GAP) to ER-Golgi v-SNAREs is an essential step for recruitment of Arf1p and coatomer, proteins that together form the COPI coat. ARF-GAP acts catalytically to recruit COPI components. Inclusion of v-SNAREs into COPI vesicles could be mediated by direct interaction with the coat. The mechanisms by which v-SNAREs interact with COPI and COPII coat proteins seem to be different and may play a key role in determining specificity in vesicle budding.

Drebrin is a novel connexin-43 binding partner that links gap junctions to the submembrane cytoskeleton.

BACKGROUND: Connexins form gap junctions that mediate the transfer of ions, metabolites, and second messengers between contacting cells. Many aspects of connexin function, for example cellular transport, plaque assembly and stability, and channel conductivity, are finely tuned and likely involve proteins that bind to connexins' cytoplasmic domains. However, little is known about such regulatory proteins. To identify novel proteins that interact with the COOH-terminal domain of Connexin-43 (Cx43), the most widely expressed connexin family member, we applied a proteomics approach to screen fractions of mouse tissue homogenates for binding partners. RESULTS: Drebrin was recovered as a binding partner of the Cx43 COOH-terminal domain from mouse brain homogenate. Drebrin had previously been described as an actin binding protein that diminishes in brains during Alzheimer's disease. The novel Drebrin-Cx43 interaction identified by proteomics was confirmed by colocalization of endogenous proteins in astrocytes and Vero cells, coimmunoprecipitation, electron microscopy, electrophysiology, coexpression of both proteins with fluorescent tags, and live-cell FRET analysis. Depletion of Drebrin in cells with siRNA results in impaired cell-cell coupling, internalization of gap junctions, and targeting of Cx43 to a degradative pathway. CONCLUSIONS: We conclude that Drebrin is required for maintaining Cx43-containing gap junctions in their functional state at the plasma membrane. It is thus possible that Drebrin may interact with gap junctions in zones of cell-cell contacts in a regulated fashion in response to extracellular signals. The rearrangement or disruption of interactions between connexins and the Drebrin-containing submembrane cytoskeleton directs connexins to degradative cellular pathways.

The alpha- and beta'-COP WD40 domains mediate cargo-selective interactions with distinct di-lysine motifs.

Coatomer is required for the retrieval of proteins from an early Golgi compartment back to the endoplasmic reticulum. The WD40 domain of alpha-COP is required for the recruitment of KKTN-tagged proteins into coatomer-coated vesicles. However, lack of the domain has only minor effects on growth in yeast. Here, we show that the WD40 domain of beta'-COP is required for the recycling of the KTKLL-tagged Golgi protein Emp47p. The protein is degraded more rapidly in cells with a point mutation in the WD40 domain of beta'-COP (sec27-95) or in cells lacking the domain altogether, whereas a point mutation in the Clathrin Heavy Chain Repeat (sec27-1) does not affect the turnover of Emp47p. Lack of the WD40 domain of beta'-COP has only minor effects on growth of yeast cells; however, absence of both WD40 domains of alpha- and beta'-COP is lethal. Two hybrid studies together with our analysis of the maturation of KKTN-tagged invertase and the turnover of Emp47p in alpha- and beta'-COP mutants suggest that the two WD40 domains of alpha- and beta'-COP bind distinct but overlapping sets of di-lysine signals and hence both contribute to recycling of proteins with di-lysine signals.

Fluorescence resonance energy transfer analysis of protein-protein interactions in single living cells by multifocal multiphoton microscopy.

Fluorescence resonance energy transfer (FRET) resolved by multifocal multiphoton microscopy (MMM) was successfully used to measure transport phenomena in living cells. We expressed different pairs of CFP-/YFP-fusion proteins involved in retrograde Golgi-to-ER transport to analyze sorting of the occupied KDEL-receptor into retrograde transport vesicles triggered by application of the external cholera toxin mutant CTXK63. FRET observed as a sensitized emission of the acceptor was confirmed by acceptor photobleaching and the dequenching of the donor was measured. FRET-MMM data obtained from single cells were compared with bulk cell experiments employing spectrofluorimetry. The importance of controlling the degree of overexpression of CFP-/YFP-fusion proteins for FRET analysis is stressed in this article. Using MMM we showed for the first time that FRET can be measured across the Golgi membrane. Finally, FRET-MMM records performed continuously over 2 h allowed to analyze intracellular retrograde transport and sorting events and to discuss these mechanisms on a single cell level.

COG complexes form spatial landmarks for distinct SNARE complexes.

Vesicular tethers and SNAREs (soluble N-ethylmalemide-sensitive fusion attachment protein receptors) are two key protein components of the intracellular membrane-trafficking machinery. The conserved oligomeric Golgi (COG) complex has been implicated in the tethering of retrograde intra-Golgi vesicles. Here, using yeast two-hybrid and co-immunoprecipitation approaches, we show that three COG subunits, namely COG4, 6 and 8, are capable of interacting with defined Golgi SNAREs, namely STX5, STX6, STX16, GS27 and SNAP29. Comparative analysis of COG8-STX16 and COG4-STX5 interactions by a COG-based mitochondrial relocalization assay reveals that the COG8 and COG4 proteins initiate the formation of two different tethering platforms that can facilitate the redirection of two populations of Golgi transport intermediates to the mitochondrial vicinity. Our results uncover a role for COG sub-complexes in defining the specificity of vesicular sorting within the Golgi.

Molecular basis for recognition of dilysine trafficking motifs by COPI.

COPI mediates retrograde trafficking from the Golgi to the endoplasmic reticulum (ER) and within the Golgi stack, sorting transmembrane proteins bearing C-terminal KKxx or KxKxx motifs. The structure of KxKxx motifs bound to the N-terminal WD-repeat domain of ß'-COP identifies electrostatic contacts between the motif and complementary patches at the center of the ß'-COP propeller. An absolute requirement of a two-residue spacing between the terminal carboxylate group and first lysine residue results from interactions of carbonyl groups in the motif backbone with basic side chains of ß'-COP. Similar interactions are proposed to mediate binding of KKxx motifs by the homologous α-COP domain. Mutation of key interacting residues in either domain or in their cognate motifs abolishes in vitro binding and results in mistrafficking of dilysine-containing cargo in yeast without compromising cell viability. Flexibility between ß'-COP WD-repeat domains and the location of cargo binding have implications for COPI coat assembly.

Two human ARFGAPs associated with COP-I-coated vesicles.

ADP-ribosylation factors (ARFs) are critical regulators of vesicular trafficking pathways and act at multiple intracellular sites. ADP-ribosylation factor-GTPase-activating proteins (ARFGAPs) are proposed to contribute to site-specific regulation. In yeast, two distinct proteins, Glo3p and Gcs1p, together provide overlapping, essential ARFGAP function required for coat protein (COP)-I-dependent trafficking. In mammalian cells, only the Gcs1p orthologue, named ARFGAP1, has been characterized in detail. However, Glo3p is known to make the stronger contribution to COP I traffic in yeast. Here, based on a conserved signature motif close to the carboxy terminus, we identify ARFGAP2 and ARFGAP3 as the human orthologues of yeast Glo3p. By immunofluorescence (IF), ARFGAP2 and ARFGAP3 are closely colocalized with coatomer subunits in NRK cells in the Golgi complex and peripheral punctate structures. In contrast to ARFGAP1, both ARFGAP2 and ARFGAP3 are associated with COP-I-coated vesicles generated from Golgi membranes in the presence of GTP-gamma-S in vitro. ARFGAP2 lacking its zinc finger domain directly binds to coatomer. Expression of this truncated mutant (DeltaN-ARFGAP2) inhibits COP-I-dependent Golgi-to-endoplasmic reticulum transport of cholera toxin (CTX-K63) in vivo. Silencing of ARFGAP1 or a combination of ARFGAP2 and ARFGAP3 in HeLa cells does not decrease cell viability. However, silencing all three ARFGAPs causes cell death. Our data provide strong evidence that ARFGAP2 and ARFGAP3 function in COP I traffic.