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애기장대 VPS29에 의한 vacuolar sorting receptor의 회귀에 관한 연구

애기장대 VPS29에 의한 vacuolar sorting receptor의 회귀에 관한 연구
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In higher plants, vacuoles occupy a large volume of the mesophyll intracellular space and play important roles in plant development and growth. There are many molecules involved in protein targeting to the vacuole. Vacuolar sorting receptors (VSRs) are key molecules for sorting vacuolar soluble cargo proteins to the vacuole. Traditionally, VSRs are considered to recognize vacuolar cargo proteins at the trans-Golgi network (TGN). Cargo receptor complexes are packaged into clathrin-coated vesicles before dissociating in the prevacuolar compartment (PVC). Free VSRs in the PVC return to the TGN for the next round of sorting of vacuolar cargoes.The retromer is involved in recycling lysosomal sorting receptors in mammals. The retromer consists of two subcomplexes, the sorting nexin dimer and a cargo-selective subcomplex. A component of the cargo-selective subcomplex in Arabidopsis, VPS29, plays a crucial role in trafficking storage proteins to protein storage vacuoles (PSVs). However, it is not known whether or how VSRs recycle from the PVC to the TGN during trafficking to the lytic vacuole (LV). An alternative model of VSR recycling has been proposed recently. In this model, VSRs bind with their cargo proteins in the endoplasmic reticulum (ER) and release cargo proteins at the TGN. Then, VSRs recycle from the TGN to the ER.This study shows that VPS29 plays an essential role in the trafficking of soluble proteins to the LV from the TGN to the PVC. The mag1-1 (maigo 1-1) mutants, which harbor a knock-down mutation in VPS29, were defective in trafficking the soluble proteins AALP:GFP and sporamin:GFP to the LV, but they were not defective in trafficking membrane proteins to the LV or plasma membrane or via the secretory pathway. In mag1-1 protoplasts, AALP:GFP and sporamin:GFP accumulated in the TGN and were secreted into the medium. In mag1-1 mutants, VSR1 failed to recycle from the PVC to the TGN
rather, a significant proportion was transported to the LV. VSR1 overexpression rescued this defect in vacuolar trafficking in mag1-1 mutants. Endogenous VSRs were expressed at higher levels in mag1-1 plants. Based on these results, I propose that VPS29 plays a crucial role in recycling VSRs from the PVC to the TGN during the trafficking of soluble proteins to the LV.
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