Structural insights for selective disruption of Beclin 1 binding to Bcl-2.

Stimulation of autophagy could provide powerful therapies for multiple diseases, including cancer and neurodegeneration. An attractive drug target for this purpose is Bcl-2, which inhibits autophagy by binding to the Beclin 1 BH3-domain. However, compounds that preclude Beclin 1/Bcl-2 binding might also induce apoptosis, which is inhibited by binding of Bcl-2 to BH3-domains of pro-apoptosis factors such as Bax. Here we describe the NMR structure of Bcl-2 bound to 35, a compound that we recently found to inhibit Beclin 1/Bcl-2 binding more potently than Bax/Bcl-2 binding. The structure shows that 35 binds at one end of the BH3-binding groove of Bcl-2. Interestingly, much of the 35-binding site is not involved in binding to Bcl-2 inhibitors described previously and mediates binding to Beclin 1 but not Bax. The structure suggests potential avenues to design compounds that disrupt Beclin 1/Bcl-2 binding and stimulate autophagy without inducing apoptosis.

Novel Bcl-2 Inhibitors Selectively Disrupt the Autophagy-Specific Bcl-2-Beclin 1 Protein-Protein Interaction.

Autophagy plays essential roles in a wide variety of physiological processes, such as cellular homeostasis, metabolism, development, differentiation, and immunity. Selective pharmacological modulation of autophagy is considered a valuable potential therapeutic approach to treat diverse human diseases. However, development of such therapies has been greatly impeded by the lack of specific small molecule autophagy modulators. Here, we performed structure-activity relationship studies on a previously discovered weak Bcl-2 inhibitor SW076956, and developed a panel of small molecule compounds that selectively released Bcl-2-mediated inhibition of autophagy-related Beclin 1 compared to apoptosis-related Bax at nanomolar concentration. Our NMR analysis showed that compound 35 directly binds Bcl-2 and specifically inhibits the interaction between the Bcl-2 and Beclin 1 BH3 domains without disruption of the Bcl-2-Bax BH3 interaction. More broadly, this proof-of-concept study demonstrates that targeting protein-protein interactions of the intrinsic autophagy regulatory network can serve as a valuable strategy for the development of autophagy-based therapeutics.

Structural and mechanistic insights into secretagogin-mediated exocytosis.

Secretagogin (SCGN) is a hexa-EF-hand protein that is highly expressed in the pancreas, brain, and gastrointestinal tract. SCGN is known to modulate regulated exocytosis in multiple cell lines and tissues; however, its exact functions and underlying mechanisms remain unclear. Here, we report that SCGN interacts with the plasma membrane SNARE SNAP-25, but not the assembled SNARE complex, in a Ca2+-dependent manner. The crystal structure of SCGN in complex with a SNAP-25 fragment reveals that SNAP-25 adopts a helical structure and binds to EF-hands 5 and 6 of SCGN. SCGN strongly inhibits SNARE-mediated vesicle fusion in vitro by binding to SNAP-25. SCGN promotes the plasma membrane localization of SNAP-25, but not Syntaxin-1a, in SCGN-expressing cells. Finally, SCGN controls neuronal growth and brain development in zebrafish, likely via interacting with SNAP-25 or its close homolog, SNAP-23. Our results thus provide insights into the regulation of SNAREs and suggest that aberrant synapse functions underlie multiple neurological disorders caused by SCGN deficiency.

Analysis of asymmetry in lipid and content mixing assays with reconstituted proteoliposomes containing the neuronal SNAREs.

Reconstitution assays with proteoliposomes provide a powerful tool to elucidate the mechanism of neurotransmitter release, but it is important to understand how these assays report on membrane fusion, and recent studies with yeast vacuolar SNAREs uncovered asymmetry in the results of lipid mixing assays. We have investigated whether such asymmetry also occurs in reconstitution assays with the neuronal SNAREs, using syntaxin-1-SNAP-25-containing liposomes and liposomes containing synaptobrevin (T and V liposomes, respectively), and fluorescent probes to monitor lipid and content mixing simultaneously. Switching the fluorescent probes placed on the T and V liposomes, we observed a striking asymmetry in both lipid and content mixing stimulated by a fragment spanning the two C2 domains of synaptotagmin-1, or by a peptide that spans the C-terminal half of the synaptobrevin SNARE motif. However, no such asymmetry was observed in assays performed in the presence of Munc18-1, Munc13-1, NSF and αSNAP, which coordinate the assembly-disassembly cycle of neuronal SNARE complexes. Our results show that switching fluorescent probes between the two types of liposomes provides a useful approach to better understand the reactions that occur between liposomes and detect heterogenous behavior in these reactions.

Multiple factors maintain assembled trans-SNARE complexes in the presence of NSF and αSNAP.

Neurotransmitter release requires formation of trans-SNARE complexes between the synaptic vesicle and plasma membranes, which likely underlies synaptic vesicle priming to a release-ready state. It is unknown whether Munc18-1, Munc13-1, complexin-1 and synaptotagmin-1 are important for priming because they mediate trans-SNARE complex assembly and/or because they prevent trans-SNARE complex disassembly by NSF-αSNAP, which can lead to de-priming. Here we show that trans-SNARE complex formation in the presence of NSF-αSNAP requires both Munc18-1 and Munc13-1, as proposed previously, and is facilitated by synaptotagmin-1. Our data also show that Munc18-1, Munc13-1, complexin-1 and likely synaptotagmin-1 contribute to maintaining assembled trans-SNARE complexes in the presence of NSF-αSNAP. We propose a model whereby Munc18-1 and Munc13-1 are critical not only for mediating vesicle priming but also for precluding de-priming by preventing trans-SNARE complex disassembly; in this model, complexin-1 also impairs de-priming, while synaptotagmin-1 may assist in priming and hinder de-priming.

Roles of the fission yeast UNC-13/Munc13 protein Ync13 in late stages of cytokinesis.

Cytokinesis is a complicated yet conserved step of the cell-division cycle that requires the coordination of multiple proteins and cellular processes. Here we describe a previously uncharacterized protein, Ync13, and its roles during fission yeast cytokinesis. Ync13 is a member of the UNC-13/Munc13 protein family, whose animal homologues are essential priming factors for soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex assembly during exocytosis in various cell types, but no roles in cytokinesis have been reported. We find that Ync13 binds to lipids in vitro and dynamically localizes to the plasma membrane at cell tips during interphase and at the division site during cytokinesis. Deletion of Ync13 leads to defective septation and exocytosis, uneven distribution of cell-wall enzymes and components of cell-wall integrity pathway along the division site and massive cell lysis during cell separation. Interestingly, loss of Ync13 compromises endocytic site selection at the division plane. Collectively, we find that Ync13 has a novel function as an UNC-13/Munc13 protein in coordinating exocytosis, endocytosis, and cell-wall integrity during fission yeast cytokinesis.

Mechanistic insights into neurotransmitter release and presynaptic plasticity from the crystal structure of Munc13-1 C1C2BMUN.

Munc13-1 acts as a master regulator of neurotransmitter release, mediating docking-priming of synaptic vesicles and diverse presynaptic plasticity processes. It is unclear how the functions of the multiple domains of Munc13-1 are coordinated. The crystal structure of a Munc13-1 fragment including its C1, C2B and MUN domains (C1C2BMUN) reveals a 19.5 nm-long multi-helical structure with the C1 and C2B domains packed at one end. The similar orientations of the respective diacyglycerol- and Ca2+-binding sites of the C1 and C2B domains suggest that the two domains cooperate in plasma-membrane binding and that activation of Munc13-1 by Ca2+ and diacylglycerol during short-term presynaptic plasticity are closely interrelated. Electrophysiological experiments in mouse neurons support the functional importance of the domain interfaces observed in C1C2BMUN. The structure imposes key constraints for models of neurotransmitter release and suggests that Munc13-1 bridges the vesicle and plasma membranes from the periphery of the membrane-membrane interface.

Sequence-specific assignment of methyl groups from the neuronal SNARE complex using lanthanide-induced pseudocontact shifts.

Neurotransmitter release depends critically on the neuronal SNARE complex formed by syntaxin-1, SNAP-25 and synaptobrevin, as well as on other proteins such as Munc18-1, Munc13-1 and synaptotagmin-1. Although three-dimensional structures are available for these components, it is still unclear how they are assembled between the synaptic vesicle and plasma membranes to trigger fast, Ca2+-dependent membrane fusion. Methyl TROSY NMR experiments provide a powerful tool to study complexes between these proteins, but assignment of the methyl groups of the SNARE complex is hindered by its limited solubility. Here we report the assignment of the isoleucine, leucine, methionine and valine methyl groups of the four SNARE motifs of syntaxin-1, SNAP-25 and synaptobrevin within the SNARE complex based solely on measurements of lanthanide-induced pseudocontact shifts. Our results illustrate the power of this approach to assign protein resonances without the need of triple resonance experiments and provide an invaluable tool for future structural studies of how the SNARE complex binds to other components of the release machinery.

Purification and characterization of amylases from small abalone (Sulculus diversicolor aquatilis).

Amylases II-1 and II-2 with molecular weights of 55.7 and 65 kDa, respectively, were purified to electrophoretical homogeneity from small abalone (Sulculus diversicolor aquatilis) by ammonium sulfate fractionation, Sepharose CL-6B, CM-Sepharose CL-6B, and Sephacryl S-100 chromatographs. They had optimal temperatures of 45 and 50 degrees C and an optimal pH of 6.0. The purified amylases were stable at pH 5.0-8.0 and 6.0-8.0, respectively. They were completely or partially inhibited by Hg(2+), Cu(2+), Cd(2+), Zn(2+), iodoacetamide, phenylmethanesulfonyl fluoride, and N-ethylmaleimide, suggesting the existence of cysteine at their active sites. Digestion tests against various polysaccharides suggested that the purified amylases II-1 and II-2 are neoamylases which can hydrolyze both alpha-1,4 and alpha-1,6 glucosidic bonds. Amylase II-2 might be an exo- and II-1 an endo-/exo-amylase.