SNAREs and membrane fusion in the Golgi apparatus.

Soluble factors, NSF and SNAPs, are required at many membrane fusion events within the cell. They interact with a class of type II integral membrane proteins termed SNAP receptors, or SNAREs. Interaction between cognate SNAREs on opposing membranes is a prerequisite for NSF dependent membrane fusion. NSF is an ATPase which will disrupt complexes composed of different SNAREs. However, there is increasingly abundant evidence that the SNARE complex recognised by NSF does not bridge the two fusing membranes, but rather is composed of SNAREs in the same membrane. The essential role of NSF may be to prime SNAREs for a direct role during fusion. The best characterised SNAREs in the Golgi are Sed5p in yeast and its mammalian homologue syntaxin 5, both of which are predominantly localised to the cis Golgi. The SNARE-SNARE interactions in which these two proteins are involved are strikingly similar. Sed5p and syntaxin 5 may mediate three distinct pathways for membrane flow into the cis Golgi, one from the ER, one from later Golgi cisternae, and possibly a third from endosomes. Syntaxin 5 is itself likely to cycle through the ER, and thus may be involved in homotypic fusion of ER derived transport vesicles. In all well characterised SNARE dependent membrane fusion events one of the interacting SNAREs is a syntaxin homologue. There are only eight members of the syntaxin family in yeast. Besides Sed5p two others, Tlg1p and Tlg2p, are found in the Golgi complex. They are present in a late Golgi compartment, but neither is required for transit of secreted proteins through the Golgi. We suggest that these observations are most compatible with a model for transit through the Golgi in which anterograde cargo is carried in cisternae, the enzymatic composition of which changes with time as Golgi resident enzymes are delivered in retrograde transport vesicles.