On the mechanism of intracellular membrane fusion: In search of the genuine fusion factor

Intracellular membrane Fusion events require a general protein machinery th at functions in vesicular traffic and in assembly and maintenance of organe lles. An array of cytosolic and integral membrane proteins are currently id entified, and in conjunction with ongoing detailed structural studies, rapi d progress is made in understanding basic features of the overall mechanism of the fusion machinery, but above all a proper appreciation of its enormo us complexity. Thus a highly sophisticated level of regulation of the diffe rent steps involved in tethering, docking and merging itself is apparent. A part from the relevance of protein protein interactions, also a role of dis tinct lipids is gradually emerging, particularly in Fusion. However, althou gh various suggestions have been made recently, largely based upon in vitro studies, the identity of the actual fusion factor(s) remains to be determi ned.