BEX5/RabA1b regulates trans-Golgi network-to-plasma membrane protein trafficking in Arabidopsis.

Constitutive endocytic recycling is a crucial mechanism allowing regulation of the activity of proteins at the plasma membrane and for rapid changes in their localization, as demonstrated in plants for PIN-FORMED (PIN) proteins, the auxin transporters. To identify novel molecular components of endocytic recycling, mainly exocytosis, we designed a PIN1-green fluorescent protein fluorescence imaging-based forward genetic screen for Arabidopsis thaliana mutants that showed increased intracellular accumulation of cargos in response to the trafficking inhibitor brefeldin A (BFA). We identified bex5 (for BFA-visualized exocytic trafficking defective), a novel dominant mutant carrying a missense mutation that disrupts a conserved sequence motif of the small GTPase, RAS GENES FROM RAT BRAINA1b. bex5 displays defects such as enhanced protein accumulation in abnormal BFA compartments, aberrant endosomes, and defective exocytosis and transcytosis. BEX5/RabA1b localizes to trans-Golgi network/early endosomes (TGN/EE) and acts on distinct trafficking processes like those regulated by GTP exchange factors on ADP-ribosylation factors GNOM-LIKE1 and HOPM INTERACTOR7/BFA-VISUALIZED ENDOCYTIC TRAFFICKING DEFECTIVE1, which regulate trafficking at the Golgi apparatus and TGN/EE, respectively. All together, this study identifies Arabidopsis BEX5/RabA1b as a novel regulator of protein trafficking from a TGN/EE compartment to the plasma membrane.

Arabidopsis RABA1 GTPases are involved in transport between the trans-Golgi network and the plasma membrane, and are required for salinity stress tolerance.

RAB GTPases are key regulators of membrane traffic. Among them, RAB11, a widely conserved sub-group, has evolved in a unique way in plants; plant RAB11 members show notable diversity, whereas yeast and animals have only a few RAB11 members. Fifty-seven RAB GTPases are encoded in the Arabidopsis thaliana genome, 26 of which are classified in the RAB11 group (further divided into RABA1-RABA6 sub-groups). Although several plant RAB11 members have been shown to play pivotal roles in plant-unique developmental processes, including cytokinesis and tip growth, molecular and physiological functions of the majority of RAB11 members remain unknown. To reveal precise functions of plant RAB11, we investigated the subcellular localization and dynamics of the largest sub-group of Arabidopsis RAB11, RABA1, which has nine members. RABA1 members reside on mobile punctate structures adjacent to the trans-Golgi network and co-localized with VAMP721/722, R-SNARE proteins that operate in the secretory pathway. In addition, the constitutive-active mutant of RABA1b, RABA1b(Q72L) , was present on the plasma membrane. The RABA1b -containing membrane structures showed actin-dependent dynamic motion . Vesicles labeled by GFP-RABA1b moved dynamically, forming queues along actin filaments. Interestingly, Arabidopsis plants whose four major RABA1 members were knocked out, and those expressing the dominant-negative mutant of RABA1B, exhibited hypersensitivity to salinity stress. Altogether, these results indicate that RABA1 members mediate transport between the trans-Golgi network and the plasma membrane, and are required for salinity stress tolerance.

New insights into the role of Arabidopsis RABA1 GTPases in salinity stress tolerance.

RAB11 GTPases, widely conserved members of RAB small GTPases, have evolved in a unique way in plants; plant RAB11 has notable diversity compared with animals and yeast. Recently, we have shown that members of RABA1, a subgroup in Arabidopsis RAB11 group, are required for salinity stress tolerance. To obtain a clue to understand its underlying mechanism, here we investigate whether RABA1 regulates sodium transport across the plasma membrane and accumulation in the vacuole. The results indicate that the raba1 quadruple mutant is not defective in the import and intracellular distribution of sodium, implying that RABA1 members are involved in a more indirect way in the responses to salinity stress.