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HOPS recognizes each SNARE, assembling ternary trans-complexes for rapid fusion upon engagement with the 4th SNARE
Yeast vacuole fusion requires R-SNARE, Q-SNAREs, and HOPS. A HOPS SM-family subunit binds the R- and Qa-SNAREs. We now report that HOPS binds each of the four SNAREs. HOPS catalyzes fusion when the Q-SNAREs are not pre-assembled, ushering them into a functional complex. Co-incubation of HOPS, proteoliposomes bearing R-SNARE, and proteoliposomes with any two Q-SNAREs yields a rapid-fusion complex with 3 SNAREs in a trans-assembly. The missing Q-SNARE then induces sudden fusion. HOPS can ‘template’ SNARE complex assembly through SM recognition of R- and Qa-SNAREs. Though the Qa-SNARE is essential for spontaneous SNARE assembly, HOPS also assembles a rapid-fusion complex between R- and QbQc-SNARE proteoliposomes in the absence of Qa-SNARE, awaiting Qa for fusion. HOPS-dependent fusion is saturable at low concentrations of each Q-SNARE, showing binding site functionality. HOPS thus tethers membranes and recognizes each SNARE, assembling R+Qa or R+QbQc rapid fusion intermediates.
Journal Article
Economics after neoliberalism
Economics is in a state of \"creative ferment,\" according to lead authors Suresh Naidu, Dani Rodrik, and Gabriel Zucman. A decade after the Great Recession, they argue for a new brand of economics, one divorced from market fundamentalism and focused instead on a more inclusive society. Responses to their ideas--which come from economists, philosophers, political scientists, and policymakers across the political spectrum--showcase just how passionate the debate over the future of economics has become. -- Publishers website
Book
Teaching Sociology
Threats to academic freedom are not new. Scholars who wrote about academic freedom decades ago were hinting at some of the same issues that we face today. However, as time passes, the threats seem to grow stronger (largely as a consequence of the increasing corporatization of higher education). While some threats are overt, and tend to flare up at particular points in time, others are of a more covert nature that slowly erode the foundations of academic freedom. This address focuses on a number of these threats (neoliberalism, contingency, political intolerance, etc.) and their relationship to sociology.
Journal Article
Sec17/Sec18 can support membrane fusion without help from completion of SNARE zippering
Membrane fusion requires R-, Qa-, Qb-, and Qc-family SNAREs that zipper into RQaQbQc coiled coils, driven by the sequestration of apolar amino acids. Zippering has been thought to provide all the force driving fusion. Sec17/αSNAP can form an oligomeric assembly with SNAREs with the Sec17 C-terminus bound to Sec18/NSF, the central region bound to SNAREs, and a crucial apolar loop near the N-terminus poised to insert into membranes. We now report that Sec17 and Sec18 can drive robust fusion without requiring zippering completion. Zippering-driven fusion is blocked by deleting the C-terminal quarter of any Q-SNARE domain or by replacing the apolar amino acids of the Qa-SNARE that face the center of the 4-SNARE coiled coils with polar residues. These blocks, singly or combined, are bypassed by Sec17 and Sec18, and SNARE-dependent fusion is restored without help from completing zippering.
Journal Article
Black-White differences in achievement: the importance of wealth
The author argues that wealth, which is an indicator of both financial and human capital, can affect the academic achievement, as well as help to explain the gap in black-white test scores. Analyses reveal that wealth affects achievement through its effect on the amount of cultural capital to which a child is exposed. Because blacks have substantially less wealth than do whites, wealth can help to explain a portion of the racial achievement gap. ... The data used in the analyses came from a subset of the data set of the National Longitudinal Survey of Youth. ... Pupils were given the Mathematics proportion of the Peabody Individual Achievement Test (PIAT). (DIPF/ Orig.)
Journal Article
Sec17 can trigger fusion of trans-SNARE paired membranes without Sec18
Sec17 [soluble N -ethylmaleimide–sensitive factor (NSF) attachment protein; α-SNAP] and Sec18 (NSF) perform ATP-dependent disassembly of cis -SNARE complexes, liberating SNAREs for subsequent assembly of trans -complexes for fusion. A mutant of Sec17, with limited ability to stimulate Sec18, still strongly enhanced fusion when ample Sec18 was supplied, suggesting that Sec17 has additional functions. We used fusion reactions where the four SNAREs were initially separate, thus requiring no disassembly by Sec18. With proteoliposomes bearing asymmetrically disposed SNAREs, tethering and trans -SNARE pairing allowed slow fusion. Addition of Sec17 did not affect the levels of trans -SNARE complex but triggered sudden fusion of trans -SNARE paired proteoliposomes. Sec18 did not substitute for Sec17 in triggering fusion, but ADP- or ATPγS-bound Sec18 enhanced this Sec17 function. The extent of the Sec17 effect varied with the lipid headgroup and fatty acyl composition of the proteoliposomes. Two mutants further distinguished the two Sec17 functions: Sec17 ᴸ²⁹¹ᴬ,ᴸ²⁹²ᴬ did not stimulate Sec18 to disassemble cis -SNARE complex but triggered the fusion of trans -SNARE paired membranes. Sec17 F²¹S,ᴹ²²S, with diminished apolar character to its hydrophobic loop, fully supported Sec18-mediated SNARE complex disassembly but had lost the capacity to stimulate the fusion of trans -SNARE paired membranes. To model the interactions of SNARE-bound Sec17 with membranes, we show that Sec17, but not Sec17 F²¹S,ᴹ²²S, interacted synergistically with the soluble SNARE domains to enable their stable association with liposomes. We propose a model in which Sec17 binds to trans -SNARE complexes, oligomerizes, and inserts apolar loops into the apposed membranes, locally disturbing the lipid bilayer and thereby lowering the energy barrier for fusion.
Significance Intracellular membrane trafficking relies on SNARE proteins from apposed membranes to form trans -complexes. Sec18 ( N -ethylmaleimide–sensitive factor; NSF) and its cochaperone Sec17 (soluble NSF attachment protein; α-SNAP) disassemble cis -SNARE complexes, liberating SNAREs for trans -complex assembly. We now describe an additional function of Sec17, its ability to trigger the fusion of trans -SNARE paired membranes. We propose a model in which Sec17 oligomerizes on trans -SNARE complexes, inserting apolar loops into the adjacent membranes. This precisely localized membrane interaction may disturb the lipid bilayer, lowering the energy barrier that prevents the two membranes from merging, and thereby facilitate fusion.
Journal Article
Membranes linked by trans-SNARE complexes require lipids prone to non-bilayer structure for progression to fusion
Like other intracellular fusion events, the homotypic fusion of yeast vacuoles requires a Rab GTPase, a large Rab effector complex, SNARE proteins which can form a 4-helical bundle, and the SNARE disassembly chaperones Sec17p and Sec18p. In addition to these proteins, specific vacuole lipids are required for efficient fusion in vivo and with the purified organelle. Reconstitution of vacuole fusion with all purified components reveals that high SNARE levels can mask the requirement for a complex mixture of vacuole lipids. At lower, more physiological SNARE levels, neutral lipids with small headgroups that tend to form non-bilayer structures (phosphatidylethanolamine, diacylglycerol, and ergosterol) are essential. Membranes without these three lipids can dock and complete trans-SNARE pairing but cannot rearrange their lipids for fusion. All cells are enclosed with a membrane that is made of phospholipid molecules, and many of the structures found inside cells—such as the vacuoles in plant and fungal cells—are also enclosed with a phospholipid membrane. To form a membrane, the phospholipid molecules—which have a phosphate head and two fatty acid tails—arrange themselves in two layers, with the fatty acid tails pointing into the membrane, and the phosphate heads pointing outwards. This structure is known as a phospholipid bilayer. Vacuoles are filled with water that contains various proteins and molecules in solution, and adjust their volume to keep the concentrations of substances in the cell in balance. To do this, the vacuoles fuse with each other. This fusion process requires dramatic spatial rearrangements of the phospholipid molecules. The SNARE family of proteins plays a key role in membrane fusion. As the two membranes come together, SNARE proteins located on each membrane form a complex known as a trans-SNARE complex. This docks the vacuole in place beside another vacuole while the phospholipid molecules in the two membranes rearrange. However, much less is known about the phospholipid molecules that are involved in the fusion process. Now, Zick et al. have shown that three types of phospholipid molecules must be present for membrane fusion to be completed. These have in common that their phosphate ‘headgroups’ are small and they do not tend to form bilayers. The vacuoles can dock beside each other if these small headgroup phospholipid molecules are not present, but the bilayer lipids in the vacuole membranes cannot rearrange themselves in the absence of these particular lipids. The importance of these nonbilayer lipid molecules had not previously been established, as the majority of experiments investigating membrane fusion used concentrations of SNARE proteins that were much higher than those found physiologically. At such high concentrations, fusion can go ahead without the nonbilayer lipid molecules being present.
Journal Article
Gendered Capital: Childhood Socialization and the “Boy Crisis” in Education
This study examined the effect of gender socialization on kindergarten grades using data from the Early Childhood Longitudinal Study- Kindergarten Cohort. The sample consisted of 6,394 children (3,177 girls; 3,217 boys) from across the United States. MANOVA and follow-up tests revealed that both boys and girls tend to participate in gender-typed activities. Girls are more likely to have positive school attitudes and exhibit positive social behavior; boys are more likely to have negative school attitudes. Regression analyses indicated that participation in “female” activities and positive social behavior positively affect grades; participation in “male” activities has no direct effect. Positive attitudes positively affect the grades of girls; negative attitudes negatively affect the grades of boys. Teacher evaluation practices are also considered.
Journal Article
Sec17/Sec18 act twice, enhancing membrane fusion and then disassembling cis-SNARE complexes
At physiological protein levels, the slow HOPS- and SNARE-dependent fusion which occurs upon complete SNARE zippering is stimulated by Sec17 and Sec18:ATP without requiring ATP hydrolysis. To stimulate, Sec17 needs its central residues which bind the 0-layer of the SNARE complex and its N-terminal apolar loop. Adding a transmembrane anchor to the N-terminus of Sec17 bypasses this requirement for apolarity of the Sec17 loop, suggesting that the loop functions for membrane binding rather than to trigger bilayer rearrangement. In contrast, when complete C-terminal SNARE zippering is prevented, fusion strictly requires Sec18 and Sec17, and the Sec17 apolar loop has functions beyond membrane anchoring. Thus Sec17 and Sec18 act twice in the fusion cycle, binding to trans-SNARE complexes to accelerate fusion, then hydrolyzing ATP to disassemble cis-SNARE complexes.
Journal Article