FUNCTIONAL CONSERVATION OF CYTOSOLIC PROTEINS REQUIRED FOR ENDOSOMAL VESICLE FUSION

Recent studies suggest that intracellular membrane traffic relies upon families of related proteins which confer specificity to individual t ransport reactions but which operate in tandem with a ubiquitous fusog enic complex containing the N-ethylmaleimide-sensitive fusion protein (NSF). The extent to which components of this process are functionally conserved is apparent from the finding that yeast Sec18 protein (Sec1 8p) can substitute for mammalian NSF in intra-Golgi transport reaction s. Here we report that yeast cytosol can support mammalian endosomal v esicle fusion, demonstrating conservation of cytosolic components requ ired for this reaction. Furthermore, under conditions in which the fus ion reaction is NSF-dependent we show that yeast Sec18p can functional ly substitute for NSF, showing that the yeast protein is capable of ca talysing at least two distinct mammalian membrane fusion events. In ad dition we exploit the complex pattern of sensitivity of the mammalian reaction to N-ethylmaleimide (NEM), coupled with the use of yeast cyto sol, to dissect a number of factors required for fusion. We reveal at least three novel NEM-sensitive activities. One of these can be restor ed by yeast cytosol suggesting that it is functionally conserved.