NSF-independent fusion mechanisms

The SNARE hypothesis provides an appealing framework for understanding the mechanism of vesicle targeting and fusion in the secretory and endocytic pathways (Rothman, 1994). In this model, vesicles bind specifically to their target membranes via complementary sets of membrane-specific SNARES (soluble N-ethylmaleimide-sensitive factor [NSF] attachment protein [SNAP] receptors). Targeting specificity is also determined by the Rabs, a family of small soluble GTPases essential for vesicle transport After specific vesicle docking is achieved, the SNARE complex recruits the soluble proteins a-SNAP, y-SNAP, and NSF. The final product of this reaction, the SNARE-SNAP-NSF complex, is proposed to be a fusion machine. NSF and the SNAPS play conserved and essential roles in membrane traffic. The ATPase activity of the trimeric NSF molecule is tightly coupled to membrane fusion in vitro(Whiteheart et al., 1994; Rothman, 1994). ATP hydro-lysis by NSF is required to dissociate detergent-solubilized SNARE-SNAP-NSF complexes in vitro (Sijllner et al., 1993). Although this is consistent with the idea that NSF (or the complex) triggers fusion directly, the identity of the fusogen(s) is not clear, and the fusion mechanism remains a mystery. The SNARE hypothesis is proposed to cover all targeted fusion events during secretion and endocytosis. This mini-review will highlight two exceptional fusion reactions in which the SNARE hypothesis may not apply. The first involves a type of fusion that was predictably distinct: homo-typic endoplasmic reticulum (ER) membrane fusion (Lat-terich and Schekman, 1994). The second example is provocative because it involves a targeted fusion event: apical vesicle transport in Madin-Darby canine kidney (MDCK) cells(lkonen et al., 1995 [this issue of Cell]). These results point to the existence of at least one NSF-independent fusion pathway in the cell. Homotypic Et? Membrane Fusion SNARE and Rab proteins ensure that vesicles fuse vecto-riallyduring secretion and endocytosis. Thus, ER transport vesicles are targeted to cis-Golgi membranes. By contrast , ER tubules can also fuse with each other in living that a dynamic balance between homotypic fission and fusion might control organelle structure and might promote organelle distribution throughout the cell. Homo-typic fission/fusion mechanisms are also thought to ensure that daughter cells inherit essential single-copy organelles during cell division. Latterich and Schekman (1994) devised an assay for ER membrane fusion in vitro, using microsomes isolated from Saccharomyces cerevisiae. Their assay measures the transfer of glycosylated pro-a factor from the lumen of microsomes that lack the glucose-trimming enzyme (glucosidase I) to the lumen of microsomes containing wild-type enzyme, …