The SNARE proteins (also called SNAREs, for short) catalyze the membrane fusion reactions in vesicle transport. There are at least 35 different SNAREs in an animal cell, each associated with a particular organelle in the secretory or endocytic pathways. These transmembrane proteins exist as complementary sets, with v-SNAREs usually found on vesicle membranes and t-SNAREs usually found on target membranes (see Figure 13–16). A v-SNARE is a single polypeptide chain, whereas a t-SNARE is usually composed of three proteins. The v-SNAREs and t-SNAREs have characteristic helical domains, and when a v-SNARE interacts with a t-SNARE, the helical domains of one wrap around the helical domains of the other to form a very stable four-helix bundle. The resulting trans-SNARE complex locks the two membranes together. Biochemical membrane fusion assays with all different SNARE combinations show that vand t-SNARE pairing is highly specific. The SNAREs thus provide an additional layer of specificity in the transport process by helping to ensure that vesicles fuse only with the correct target membrane.
The trans-SNARE complexes catalyze membrane fusion by using the energy that is freed when the interacting helices wrap around each other to pull the membrane faces together, simultaneously squeezing out water molecules from the interface (Figure 13–19). When liposomes containing purified v-SNAREs are mixed with liposomes containing complementary t-SNAREs, their membranes fuse, albeit slowly. In the cell, other proteins recruited to the fusion site, presumably Rab effectors, cooperate with SNAREs to accelerate fusion. Fusion does not always follow immediately after v-SNAREs and t-SNAREs pair. As we discuss later, in the process of regulated exocytosis, fusion is delayed until secretion is triggered by a specific extracellular signal.
Rab proteins, which can regulate the availability of SNARE proteins, exert an additional layer of control. t-SNAREs in target membranes are often associated with inhibitory proteins that must be released before the t-SNARE can function. Rab proteins and their effectors trigger the release of such SNARE inhibitory
Figure 13–19 A model for how SNARE proteins may catalyze membrane fusion. bilayer fusion occurs in multiple steps. a tight pairing between vand t-snares forces lipid bilayers into close apposition and expels water molecules from the interface. lipid molecules in the two interacting (cytosolic) leaflets of the bilayers then flow between the membranes to form a connecting stalk. lipids of the two noncytosolic leaflets then contact each other, forming a new bilayer, which widens the fusion zone (hemifusion, or half-fusion). rupture of the new bilayer completes the fusion reaction.
Figure 13–18 A model for a generic Rab cascade. The local activation of a raba-gef leads to assembly of a raba domain on the membrane. active raba recruits its effector proteins, one of which is a gef for rabb. The rabb-gef then recruits rabb to the membrane, which in turn begins to recruit its effectors, among them a gap for raba. The raba-gap activates raba gTp hydrolysis leading to the inactivation of the raba and the disassembly of the raba domain as the rabb domain grows. In this way, the raba domain is irreversibly replaced by the rabb domain. In principle, this sequence can continued by the recruitment of a next gef by rabb. (adapted from a.h. hutagalung and p.J. novick, Physiol. rev. 91:119–149, 2011. with permission from The american physiological society.) proteins. In this way, SNARE proteins are concentrated and activated in the correct location on the membrane, where tethering proteins capture incoming vesicles. Rab proteins thus speed up the process by which appropriate SNARE proteins in two membranes find each other.