Small GTP-binding proteins are monomeric G proteins with molecular masses of 20–40 kDa. Small G proteins in eukaryotes from yeast to human constitute a superfamily with at least five families including more than 100 members. Although plants have only four of these families of small G proteins, they have a unique subfamily of Rho GTPases instead of the Ras family. Rac1, a member of the Rho family, has been shown to play an essential role in the defense of rice against pathogens. The functions of other superfamilies of small G proteins in plant toward pathogens have not been determined. The Rab proteins belong to the small guanosine triphosphatases superfamily. Rabs are thought to act as molecular switches, which play an essential role in both endocytic and exocytic traffic in eukaryotic cells, being active in their GTPbound state and inactive in their GDC-0879 GDP-bound state.
Because Rabs do not have high intrinsic guanine nucleotide exchange or hydrolytic activities, they are regulated by other proteins, such as guanine nucleotide exchange factors and GTPase-activating proteins. In their GDP-bound state, Rabs are typically soluble and bound to guanine nucleotide dissociation inhibitor. At the acceptor membrane, the Rab-GDI complex is thought to interact with GDI displacement factor, which removes GDI and allows Rab membrane insertion. Next, a GEF converts Rab to its GTP-bound, active conformation, allowing it to interact with its downstream effectors. Rabs regulate cell proliferation, cytoskeleton organization, intracellular membrane trafficking and vesicle motility along the actin/microtubule cytoskeletons, vesicle tethering, transport, and fusion. Many downstream effectors of Rab7 in mammals have been extensively characterized and shown to interact with their partners to exert biological functions. Rab7 is a key regulator in the process of phagosome maturation. Rab7 and the Rab interacting lysosomal protein are essential factors in regulating the maturation of the phagosome into a lysophagosome. In addition, the homotypic fusion and vacuole protein sorting may play dual roles as upstream GEF and downstream tethering effector of Rab7 to facilitate endosomal membrane fusion.
Approximately 70 members have been identified in mammals, and Rab7 is one of the Rab proteins that have been investigated extensively. Rab7 is regarded as a key regulator in endo-lysosomal trafficking based on the extensive investigations in the past decades. Rab7 mediates the regulated internalization and degradation of nutrient transporters and triggers nutrient starvation that facilitates cell death. A tonoplast-localized rice Rab7 homologue is up-regulated in response to cold, salt, and drought stress, suggesting that it plays a role in plant adaptation to various environmental stresses. In Arabidopsis, Kwon et al. analyzed a SA-responsive protein involved in plant-pathogen interactions RabG3b, which shows strong similarity to the Rab7 in mammals and yeast which contain conserved motifs for GTP binding and hydrolysis. Compared to wild-type and transgenic plants overexpressing dominant negative RabG3b, transgenic plants overexpressing.