Vesicle transport in plants: A revised phylogeny of SNARE proteins
Communication systems within and between plant cells involve the transfer of ions and molecules between compartments, and are essential for development and responses to biotic and abiotic stresses. This is turn requires the regulated movement and fusion of membrane systems with their associated cargo. Recent advances in genomics has provided new resources with which to investigate the evolutionary relationships between membrane proteins across plant species. Results Members of the soluble
... f the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are known to play important roles in vesicle trafficking across plant, animal and microbial species. Using recent public expression and transcriptomic data from nine representative green plants, we investigated the evolution of the SNARE classes and linked protein changes to functional specialization (expression patterns). We identified an additional three putative SNARE genes in the model plant Arabidopsis. We found that all SNARE classes have expanded in number to a greater or lesser degree alongside the evolution of multicellularity. Sub-functionalization of SNARE family members to different tissues, associated with an accumulation of protein changes is typical, although at least one example of neo-functionalization between the R-SNAREs VAMP722 and VAMP723 was also observed. Conclusion These results provide an insight into SNARE protein evolution and functional specialization. The work provides a platform for hypothesis-building and future research into the precise functions of these proteins in plant development and responses to the environment. Background Communication between cells, and between compartments within cells, depends on membrane structure and function, and is fundamental to plant and animal growth and development and responses to environmental stresses. The regulated dynamics of membranes, and the associated proteins, is mediated by the membrane trafficking system, and is of great current interest to those 3 interested in understanding the cell as an integrated system of signals and molecular responses. The membrane trafficking system in plant cells comprises the biosynthetic secretory pathway, the endocytic pathway and the vesicle transport pathway  . The organelles involved in these processes use small membrane-enclosed transport vesicles to exchange molecular information. There are four essential steps in the vesicle trafficking system, namely budding, vesicle movement, tethering and fusion (Fig. 1) . During budding, coat proteins and dynamin-related GTPases in a donor compartment are used to form a vesicle and deform the local membrane until a vesicle is freed by scission. Cargo and vesicle (v)-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are incorporated into the budding vesicle by binding to coat subunits. In the movement phase, the freed vesicle moves towards the acceptor compartment by association with cytoskeletal motors. Molecular motors including kinesin, dynein, and myosin have all been shown to be involved in this process [2, 3] . Then tethering and docking factors work in conjunction with Rab GTPases and SNARE proteins to tether the vesicle to their acceptor membrane  . In the final,fusion step, a tetrameric trans (t)-SNARE complex is formed from a single v-SNARE molecule (members of the synaptobrevin or VAMP family of proteins) and a trimeric target membrane t-SNARE complex (members of the syntaxin and SNAP-25 families) that allows vesicles to identify their target compartment and complete membrane fusion and cargo delivery      .