Clostridial neurotoxin-insensitive vesicular SNAREs in exocytosis and endocytosis

Abstract

The SNARE model of membrane fusion was proposed by Rothman and coworkers following their discovery of a protein complex linking synaptic vesicles to the neuronal plasma membrane (Sollner et al., 1993a,b). In the original model, synaptobrevin 2 (also called Vesicle Associated Membrane Protein 2, or VAMP2), a synaptic vesicle protein, and syntaxin 1 and Synaptosomal Associated Protein of 25 kD (SNAP25), two neuronal plasma membrane proteins, were proposed to mediate membrane fusion through the formation of a complex. This hypothesis has now been largely confirmed and generalised. Indeed, SNARE proteins have been identified in all organisms from yeast and mammals, and they are involved in all intracellular membrane fusion events, thus allowing for vesicle trafficking, cell polarisation and plasma membrane expansion (for review see Jahn and Sudhof, 1999). In the most recent period, key experiments have allowed to answer important questions raised by the original SNARE model. Indeed, the elucidation of the structure of the complex showing parallel orientation of vesicle (v–) and target (t–) SNAREs (Sutton et al., 1998), the demonstration that SNAREs are not only necessary but also sufficient for lipid bilayer fusion (Weber et al., 1998), together with the recent results showing that a significant degree of specificity is encoded in SNARE proteins themselves (McNew et al., 2000; Scales et al., 2000) have helped to strengthen and refine the SNARE model. The pioneering work of the groups of H. Niemann and C. Montecucco demonstrated that the pre-synaptic SNARE proteins, syntaxin 1, SNAP25, and synaptobrevin 2, are the targets of clostridial neurotoxins. This property has been crucial to define their function in several studies (for review see Niemann et al., 1994; Schiavo et al., 1994; Humeau et al., 2000). Most importantly, the first evidence for a role of a SNARE protein in exocytosis was deduced from the association of tetanus neurotoxin-induced block of secretion with its proteolytic activity on synaptobrevin 2 (Link et al., 1992; Schiavo et al., 1992). Nevertheless, several transport pathways are either resistant or only partially affected by clostridial neurotoxins. Among those, apical transport in polarised cells (Ikonen et al., 1995), and axonal outgrowth in neurons (Osen-Sand et al., 1996) are not inhibited by tetanus neurotoxin (for review see Johannes and Galli, 1998). The recent identification of neurotoxin-resistant SNAREs allowed to reconcile the lack of effect of clostridial neurotoxins with the implication of the SNARE proteins in all known membrane fusions. The aim of this review is to briefly discuss the latest studies on the tetanus neurotoxin-insensitive VAMP (TI-VAMP, also known as VAMP7), and endobrevin (also called VAMP8), two clostridial neurotoxinresistant v–SNAREs.