They are then transported into the PSVs where the precursors are converted to mature forms. Two distinct pathways have been identified for the trafficking of storage proteins to the PSVs: the Golgi-dependent and -independent pathways.Resatorvid In the Golgi-dependent pathway, proteins are transported through the Golgi and then dense vesicles before being delivered into the PSVs. In the Golgi-independent pathway, however, proteins are transported from the ER to the PSVs through the precursor-accumulating vesicles.The molecular machinery involved in protein trafficking to the PSVs is beginning to be explored. VSR1, VSR3 and VSR4 have been shown to function as the vacuolar sorting receptors of the PSVs in Arabidopsis. RMR1, the receptor homology region transmembrane domain ring H2 motif protein, is another type of sorting receptors for the trafficking to the PSVs in Arabidopsis. PV72 was reported to be the pumpkin VSR on the membranes of the PAC vesicles. AtVPS29, a member of the retromer complex that recycles VPS10 from the prevacuolar compartment to the Golgi, is involved in recycling VSRs for the sorting of storage proteins. In addition, the SNARE complex composed of VAMP727, SYP22, VTI11, and SYP51, which mediates the fusion between the PVC and the vacuole, is crucial for protein transport into the PSVs in Arabidopsis.Cellular pH is a key regulatory factor of protein trafficking in both the secretory and endocytic pathways. Studies have shown that the organelles become more acidic along the process of maturation in the exocytic or endocytic pathways in both the plant and animal cells. In yeast and animals, the acidity of the organelles is generated by the vacuolar-type H+-ATPases. Moreover, Na+/H+ antiporters in animals conduct proton leak to counter organelle acidification in order to maintain an optimal pH. In plants, however, the acidic pH of the organelles is maintained by the proton pumps V-ATPases and pyrophosphatase. Similar to their animal counterparts, the Golgi/TGN-localized plant NHX antipoters, AtNHX5 and AtNHX6, may act as a H+-leak system to counter the luminal acidification.Studies have shown that protein trafficking to the PSVs is controlled by pH and requires V-ATPases and NHX antiporters: The binding of the VSR to its ligand is pH-dependent; the binding occurs from pH 4.0 to 7.0, with an optimal binding at pH 6.0; V-ATPase is required for the sorting of soluble vacuolar protein precursors in tobacco cells; The antibiotic Na+/H+ antiporter monensin, which causes the acidification of the TGN, affects the vacuolar transport of the seed storage proteins; and AtNHX5 and AtNHX6 are required for the transport of the seed storage proteins into the vacuole as well as processing of the seed storage proteins in Arabidopsis.Plant NHX antiporters are membrane proteins that transport protons across a membrane in exchange for Na+ or K+. Studies show that plant NHX antiporters are critical for cellular ion homeostasis and pH regulation, and play significant roles in diverse cellular processes, including pH homeostasis, Na+ and K+ movement, vesicle trafficking and fusion, growth and development, and salt tolerance.Torin The Arabidopsis NHX gene family contains 8 members that are divided into three distinct classes: vacuolar , endosomal , and plasma membrane. AtNHX5 and AtNHX6 are localized in the Golgi, TGN, and PVC, and share high sequence similarity . AtNHX5 and AtNHX6 play an important role in plant growth and development. nhx5 nhx6 double mutant showed profound defects in growth and development. nhx5 nhx6 had smaller rosettes and shorter seedlings, and was flowering and bolting late.
