Ral and Rab3a are major GTP-binding proteins of axonal rapid transport and synaptic vesicles and do not redistribute following depolarization stimulated synaptosomal exocytosis

Rab3 proteins: key players in the control of exocytosis

Although some mechanistic aspects of exocytosis, such as fusion events, have been well documented by the technique of time-resolved membrane-capacitance measurement, it was only recently that new insights into the molecular mechanisms involved in the traffic of secretory vesicles were provided by the convergence of different lines of research. In this review Lledo et al. present some of the recent findings concerning small GTPases of the Rab3 subfamily which regulate hormone release, triggered by entry of Ca2+, in endocrine and neuroendocrine cells. In view of these new results, Rab proteins might be considered as candidates for inhibition or stimulation of specific steps involved in vesicle traffic.

Role of the Rab3A GTPase in regulated secretion from neuroendocrine cells

Regulated secretion is a complex process that involves scores of distinct proteins. These proteins must assemble into machines that organize vesicle docking, membrane fusion, and recovery of vesicle components. Rab3A, a member of the Ras superfamily of GTPases, associates with secretory granule membranes of neuroendocrine cells, and mutants of Rab3A can inhibit exocytosis. A model is proposed in which Rab3A controls the assembly and disassembly of a docking complex that inhibits membrane fusion until an external signal triggers release of the Rab3A from the complex.

The exocytotic fusion pore and neurotransmitter release

Membrane fusion is ubiquitous in biological systems, occurring in the simplest of unicellular eukaryotes as well as higher eukaryotes. As soon as the first primitive eukaryotic cell utilized a lipid bilayer as an outer membrane, membrane fusion (and fission) became necessary for the traffic of material from the outside to the inside, the inside to the outside, and between different intracellular membrane-bounded compartments. The earliest cells would have made use of the intrinsic ability of lipid bilayers to fuse under certain conditions. Although this fusogenic property of bilayers has been known for some time, it is has become clear only relatively recently that two phospholipid bilayers will fuse spontaneously, owing to a hydrophobic force, when the bilayers are brought close together under conditions of membrane tension or high curvature (Helm and Israelachvili, 1993). The primeval cell would have used proteins to develop the appropriate architecture in which such fusion would occur in a regulated manner. During the course of evolution, ever more sophisticated ways of regulating this basic process would evolve, but the underlying fusion mechanism would remain unchanged. We have proposed that a macromolecular scaffold of proteins is responsible for bringing the plasma membrane close to the secretory granule membranes and creating the architecture that enables the hydrophobic force to cause fusion (Figure 1; Nanavati et al., 1992; Monck and Fernandez, 1992; Oberhauser and Fernandez, 1993). Evidence is now accumulating that there are several highly conserved families of proteins associated with vesicle fusion events, from yeast to mammalian cells, and with intracellular traffic, as well as with regulated exocytosis and synaptic transmission (Bennett and Scheller, 1993; Sollner et al., 1993; Südhof et al., 1993). The molecular structures (or scaffolds) that regulate membrane fusion are likely to contain related proteins and share certain fundamental properties.