Putative fusogenic activity of NSF is restricted to a lipid mixture whose coalescence is also triggered by other factors

It has recently been reported that N-ethylmaleimidesensitive fusion ATPase (NSF) can fuse protein-free liposomes containing substantial amounts of 1,2 -dioleoylphosphatidylserine (DOPS) and 1,2-dioleoyl-phosphatidyl-ethanolami ne (DOPE) (Otter-Nilsson et al., 1999), The authors impart physiological si gnificance to this observation and propose to re-conceptualize the general role of NSF in fusion processes. We can confirm that isolated NSF can fuse liposomes of the specified composition. However, this activity of NSF is re sistant to inactivation by N-ethylmaleimide and does not depend on the pres ence of alpha-SNAP (soluble NSF-attachment protein). Moreover, under the sa me conditions, either alpha-SNAP, other proteins apparently unrelated to ve sicular transport (glyceraldehyde-3-phosphate dehydrogenase or lactic dehyd rogenase) or even 3 mM magnesium ions can also cause lipid mixing. In contr ast, neither NSF nor the other proteins nor magnesium had any significant f usogenic activity with liposomes composed of a biologically occurring mixtu re of lipids. A straightforward explanation is that the lipid composition c hosen as optimal for NSF favors non-specific fusion because it is physicall y unstable when formed into liposomes, A variety of minor perturbations cou ld then trigger coalescence.