Chemical proteomics reveals bolinaquinone as a clathrin-mediated endocytosis inhibitor

The emerging field of mass spectrometry-based chemical proteomics provides a powerful instrument in the target discovery of bioactive small-molecules, such as drugs or natural products. The identification of their macromolecular targets is required for a comprehensive understanding of their bio-pharmacological role and for unraveling their mechanism of action. We report the application of a chemical proteomics approach to the analysis of the cellular interactome of the marine metabolite bolinaquinone (BLQ). BLQ was linked to an opportune α,ω-diamino polyethylene glycol chain and then immobilized on a matrix support. The modified beads were then used as a bait for fishing the potential partners of BLQ in a THP-1 macrophage cell lysate. Surprisingly, we identified clathrin, a protein involved in the cell internalization of proteins, viruses and other biologically relevant macromolecules, as a specific and major BLQ partner. In addition, we verified the biochemical role of BLQ testing its ability to inhibit the clathrin-mediated endocytosis of albumin. This finding indicates BLQ as a new biotechnological tool for cell endocytosis studies and paves the way to further investigation on its potential role in modulating internalization process.     

Conservation and specialization. The role of the exocyst in neuronal exocytosis

The exocyst is an octameric complex mediating vesicle targeting and tethering at the plasma membrane for exocytosis. The role of exocyst in nervous system is unclear. In this issue of Neuron, Murthy et al. provide important insights: defects in the exocyst inhibit neurite outgrowth and neuromuscular junction formation; however, synaptic transmission persists.     

Phorbol myristate acetate induces endocytosis as well as exocytosis and hydroosmosis in toad urinary bladder

The induction of the hydroosmotic response in the toad urinary bladder is considered to be associated with membrane addition mediated by exocytosis at the affected luminal membrane and reversed by endocytic retrieval at that surface. The permeability, exocytosis and endocytosis are initiated by antidiuretic hormone (ADH) receptor interaction on the basolateral membrane. In other hormone responsive systems, phorbol ester (phorbol myristate acetate, PMA), a tumor promoter, has been implicated in the regulation of various transport processes through the activation of protein kinase C and cytoskeletal protein phosphorylation. We found that addition of 10(-6) M PMA to the mucosa induces an hydroosmotic response which is gradual and which reaches a maximum within 60 min, equal to about 1/3 the maximal ADH response. Morphologically, PMA causes rapid exocytosis of the granules, endocytosis of horseradish peroxidase from the mucosal medium into tubules and multivesicular bodies and elongation of apical microvilli. Controls treated with mucosal 0.1% dimethylsulfoxide (DMSO) or an inactive PMA isomer on the mucosal surface, or PMA on the serosal surface lack the hydroosmotic, exocytic, endocytic and cytoskeletal changes. Addition of serosal ADH to PMA-treated bladders results in a precocious hydroosmotic and exocytic ADH response, but a lowering of the maximal response. Also pretreatment of bladders with PMA prevented the ADH-induced increase in transepithelial potential difference. Thus, apical events mediating the PMA hydroosmotic response are correlated with exo- and endocytosis and elongation of apical microvilli.     

Elimination of a bacterial pore-forming toxin by sequential endocytosis and exocytosis

Staphylococcus aureus α-toxin is the archetype of bacterial pore forming toxins and a key virulence factor secreted by the majority of clinical isolates of S. aureus. Toxin monomers bind to target cells and oligomerize to form small β-barrel pores in the plasma membrane. Many nucleated cells are able to repair a limited number of lesions by unknown, calcium-independent mechanisms. Here we show that cells can internalize α-toxin, that uptake is essential for cellular survival, and that pore-complexes are not proteolytically degraded, but returned to the extracellular milieu in the context of exosome-like structures, which we term toxosomes.     

DnaK functions as a central hub in the E. coli chaperone network

Cellular chaperone networks prevent potentially toxic protein aggregation and ensure proteome integrity. Here, we used Escherichia coli as a model to understand the organization of these networks, focusing on the cooperation of the DnaK system with the upstream chaperone Trigger factor (TF) and the downstream GroEL. Quantitative proteomics revealed that DnaK interacts with at least ~700 mostly cytosolic proteins, including ~180 relatively aggregation-prone proteins that utilize DnaK extensively during and after initial folding. Upon deletion of TF, DnaK interacts increasingly with ribosomal and other small, basic proteins, while its association with large multidomain proteins is reduced. DnaK also functions prominently in stabilizing proteins for subsequent folding by GroEL. These proteins accumulate on DnaK upon GroEL depletion and are then degraded, thus defining DnaK as a central organizer of the chaperone network. Combined loss of DnaK and TF causes proteostasis collapse with disruption of GroEL function, defective ribosomal biogenesis, and extensive aggregation of large proteins.     

Protein scaffolds in the coupling of synaptic exocytosis and endocytosis

Mechanisms that ensure robust long-term performance of synaptic transmission over a wide range of activity are crucial for the integrity of neuronal networks, for processing sensory information and for the ability to learn and store memories. Recent experiments have revealed that such robust performance requires a tight coupling between exocytic vesicle fusion at defined release sites and endocytic retrieval of synaptic vesicle membranes. Distinct presynaptic scaffolding proteins are essential for fulfilling this requirement, providing either ultrastructural coordination or acting as signalling hubs.     

Regulation of exocytosis by the exocyst subunit Sec6 and the SM protein Sec1

Trafficking of protein and lipid cargo through the secretory pathway in eukaryotic cells is mediated by membrane-bound vesicles. Secretory vesicle targeting and fusion require a conserved multisubunit protein complex termed the exocyst, which has been implicated in specific tethering of vesicles to sites of polarized exocytosis. The exocyst is directly involved in regulating soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) complexes and membrane fusion through interactions between the Sec6 subunit and the plasma membrane SNARE protein Sec9. Here we show another facet of Sec6 function-it directly binds Sec1, another SNARE regulator, but of the Sec1/Munc18 family. The Sec6-Sec1 interaction is exclusive of Sec6-Sec9 but compatible with Sec6-exocyst assembly. In contrast, the Sec6-exocyst interaction is incompatible with Sec6-Sec9. Therefore, upon vesicle arrival, Sec6 is proposed to release Sec9 in favor of Sec6-exocyst assembly and to simultaneously recruit Sec1 to sites of secretion for coordinated SNARE complex formation and membrane fusion.     

Lethal giant larvae proteins interact with the exocyst complex and are involved in polarized exocytosis

The tumor suppressor lethal giant larvae (Lgl) plays a critical role in epithelial cell polarization. However, the molecular mechanism by which Lgl carries out its functions is unclear. In this study, we report that the yeast Lgl proteins Sro7p and Sro77p directly interact with Exo84p, which is a component of the exocyst complex that is essential for targeting vesicles to specific sites of the plasma membrane for exocytosis, and that this interaction is important for post-Golgi secretion. Genetic analyses demonstrate a molecular pathway from Rab and Rho GTPases through the exocyst and Lgl to SNAREs, which mediate membrane fusion. We also found that overexpression of Lgl and t-SNARE proteins not only improves exocytosis but also rescues polarity defects in exocyst mutants. We propose that, although Lgl is broadly distributed in the cells, its localized interaction with the exocyst and kinetic activation are important for the establishment and reenforcement of cell polarity.     

Genomewide screen reveals a wide regulatory network for di/tripeptide utilization in Saccharomyces cerevisiae

Small peptides of two to six residues serve as important sources of amino acids and nitrogen required for growth by a variety of organisms. In the yeast Saccharomyces cerevisiae, the membrane transport protein Ptr2p, encoded by PTR2, mediates the uptake of di/tripeptides. To identify genes involved in regulation of dipeptide utilization, we performed a systematic, functional examination of this process in a haploid, nonessential, single-gene deletion mutant library. We have identified 103 candidate genes: 57 genes whose deletion decreased dipeptide utilization and 46 genes whose deletion enhanced dipeptide utilization. On the basis of Ptr2p-GFP expression studies, together with PTR2 expression analysis and dipeptide uptake assays, 42 genes were ascribed to the regulation of PTR2 expression, 37 genes were involved in Ptr2p localization, and 24 genes did not apparently affect Ptr2p-GFP expression or localization. The 103 genes regulating dipeptide utilization were distributed among most of the Gene Ontology functional categories, indicating a very wide regulatory network involved in transport and utilization of dipeptides in yeast. It is anticipated that further characterization of how these genes affect peptide utilization should add new insights into the global mechanisms of regulation of transport systems in general and peptide utilization in particular.     

Botulinum neurotoxin A blocks synaptic vesicle exocytosis but not endocytosis at the nerve terminal

The supply of synaptic vesicles in the nerve terminal is maintained by a temporally linked balance of exo- and endocytosis. Tetanus and botulinum neurotoxins block neurotransmitter release by the enzymatic cleavage of proteins identified as critical for synaptic vesicle exocytosis. We show here that botulinum neurotoxin A is unique in that the toxin-induced block in exocytosis does not arrest vesicle membrane endocytosis. In the murine spinal cord, cell cultures exposed to botulinum neurotoxin A, neither K(+)-evoked neurotransmitter release nor synaptic currents can be detected, twice the ordinary number of synaptic vesicles are docked at the synaptic active zone, and its protein substrate is cleaved, which is similar to observations with tetanus and other botulinal neurotoxins. In marked contrast, K(+) depolarization, in the presence of Ca(2+), triggers the endocytosis of the vesicle membrane in botulinum neurotoxin A-blocked cultures as evidenced by FM1-43 staining of synaptic terminals and uptake of HRP into synaptic vesicles. These experiments are the first demonstration that botulinum neurotoxin A uncouples vesicle exo- from endocytosis, and provide evidence that Ca(2+) is required for synaptic vesicle membrane retrieval.     

物质的内吞、外排与质膜、内膜系统的关系

高中生物必修本第1册第2章在讲到细胞膜的主要功能时,以小字的形式提到细胞的内吞作用和外排作用,表述较简单,读者不甚理解。其实这也是活细胞进行新陈代谢作用,不断地与外界环境交换物质,物质通过细胞膜进出细胞的方式之一。离子和小分子物质进出     

A mutant screen reveals RNase E as a silencer of group II intron retromobility in Escherichia coli

Group II introns are mobile retroelements that invade their hosts. The Lactococcus lactis group II intron recruits cellular polymerases, nucleases, and DNA ligase to complete the retromobility process in Escherichia coli. Here we describe a genetic screen with a Tn5 transposon library to identify other E. coli functions involved in retromobility of the L. lactis LtrB intron. Thirteen disruptions that reproducibly resulted in increased or decreased retrohoming levels into the E. coli chromosome were isolated. These functions were classified as factors involved in RNA processing, DNA replication, energy metabolism, and global regulation. Here we characterize a novel mutant in the rne promoter region, which regulates RNase E expression. Retrohoming and retrotransposition levels are elevated in the rneTn5 mutant. The stimulatory effect of the mutation on retromobility results from intron RNA accumulation in the RNase E mutant. These results suggest that RNase E, which is the central component of the RNA degradosome, could regulate retrohoming levels in response to cellular physiology.     

A hemifused complex is the hub in a network of pathways to membrane fusion

Membrane fusion is a critical step for many essential processes, from neurotransmission to fertilization. For over 40 years, protein-free fusion driven by calcium or other cationic species has provided a simplified model of biological fusion, but the mechanisms remain poorly understood. Cation-mediated membrane fusion and permeation are essential in their own right to drug delivery strategies based on cell-penetrating peptides or cation-bearing lipid nanoparticles. Experimental studies suggest calcium drives anionic membranes to a hemifused intermediate that constitutes a hub in a network of pathways, but the pathway selection mechanism is unknown. Here we develop a mathematical model that identifies the network hub as a highly dynamic hemifusion complex. Multivalent cations drive expansion of this high-tension hemifusion interface between interacting vesicles during a brief transient. The fate of this interface determines the outcome, either fusion, dead-end hemifusion, or vesicle lysis. The model reproduces the unexplained finding that calcium-driven fusion of vesicles with planar membranes typically stalls at hemifusion, and we show the equilibrated hemifused state is a novel lens-shaped complex. Thus, membrane fusion kinetics follow a stochastic trajectory within a network of pathways, with outcome weightings set by a hemifused complex intermediate.     

Quantitative analysis of exocytosis and endocytosis in the hydroosmotic response of toad bladder

Summary This study concerns the timing and magnitude of exocytosis and endocytosis in the granular cells of toad bladder during the hydroosmotic response to antidiuretic hormone. Granule exocytosis at the luminal cell surface is extensive within 5 min of the administration of a physiological dose of hormone. Hydroosmosis becomes detectable during this time period. The amount of membrane added to the luminal surface by exocytosis during 60 min of exposure to hormone can be of the same order of magnitude as the extent of the luminal plasma membrane. Endocytosis, demonstrated by peroxidase uptake from the luminal surface, becomes extensive during the period 15–45 min after hormone administration. Thus, maximal endocytic activity occurs later than the period of most extensive exocytosis and seems to correlate with the onset of the decline in water movement. The amount of membrane retrieved from the luminal surface by endocytosis during 60 min of stimulation is at least three quarters of that added by exocytosis. The bulk membrane movement in ADH stimulated preparations does not require the presence of an osmotic gradient. Colchicine inhibits the hydroosmotic response, the exocytosis of granules, and endocytosis at the luminal surface. These results strengthen our view that the bulk circulation of membrane at the cell surface, via exocytosis and endocytosis, is closely related to the permeability changes occuring at the surface.     

Rho GTPase regulation of exocytosis in yeast is independent of GTP hydrolysis and polarization of the exocyst complex

Rho GTPases are important regulators of polarity in eukaryotic cells. In yeast they are involved in regulating the docking and fusion of secretory vesicles with the cell surface. Our analysis of a Rho3 mutant that is unable to interact with the Exo70 subunit of the exocyst reveals a normal polarization of the exocyst complex as well as other polarity markers. We also find that there is no redundancy between the Rho3-Exo70 and Rho1-Sec3 pathways in the localization of the exocyst. This suggests that Rho3 and Cdc42 act to polarize exocytosis by activating the exocytic machinery at the membrane without the need to first recruit it to sites of polarized growth. Consistent with this model, we find that the ability of Rho3 and Cdc42 to hydrolyze GTP is not required for their role in secretion. Moreover, our analysis of the Sec3 subunit of the exocyst suggests that polarization of the exocyst may be a consequence rather than a cause of polarized exocytosis.     

Calcium dependence of exocytosis and endocytosis at the cochlear inner hair cell afferent synapse

Release of neurotransmitter at the inner hair cell (IHC) afferent synapse is a fundamental step in translating sound into auditory nerve excitation. To study the Ca2+ dependence of the underlying vesicle fusion and subsequent endocytosis, we combined Ca2+ uncaging with membrane capacitance measurements in mouse IHCs. Rapid elevations in [Ca2+]i above 8 microM caused a biphasic capacitance increase corresponding to the fusion of approximately 40,000 vesicles. The kinetics of exocytosis displayed a fifth-order Ca2+ dependence reaching maximal rates of >3 x 10(7) vesicle/s. Exocytosis was always followed by slow, compensatory endocytosis (tau congruent with 15 s). Higher [Ca2+]i increased the contribution of a faster mode of endocytosis with a Ca2+ independent time constant of approximately 300 ms. These properties provide for rapid and sustained transmitter release from this large presynaptic terminal.