A Key Regulator of Post-Golgi Vesicular Traffic

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Ren et al. identify a downstream effector of Rab5a that associates with the tethering and fusion complexes to mediate membrane fusion of dense vesicles with the protein storage vacuoles.

Plant Cell https://doi.org/10.1105/tpc.19.00863

By Y.L. Ren, Y.H. Wang and T. Pan

Background: Plants use a special type of vacuole, the protein storage vacuoles (PSVs), to store reserve proteins within seeds. Dense vesicles (DVs) devoid of a recognizable protein coat define a plant-unique carrier for post-Golgi trafficking of storage proteins to the PSVs. We used a forward genetic approach to identify mutants defective in glutelin (a major storage protein in rice) sorting to the PSVs. Our previous studies showed that GPA1 and GPA2, encoding the small GTPase Rab5a and its guanine exchange factor VPS9a, respectively, play important roles in the directional targeting of DVs to the PSVs in rice endosperm.

Question: Small GTPases usually function via evoking their downstream effectors. We aimed to isolate effectors of GPA1/Rab5a and determine how they regulate the directional targeting of DVs to the PSVs.

Findings: We identified a rice mutant named glutelin precursor accumulation5 (gpa5), which exhibits a defect in the directional targeting of DVs to the PSVs evidenced by the missorting of glutelins into the extracellular space. We cloned gpa5 and identified the causal gene as a plant-unique lipid-binding protein GPA5. The GPA5 protein can form homodimers and is localized to the periphery of DVs. Interestingly, we found that GPA5 specifically interacts with the active form of GPA1/Rab5a, and that GPA5 associates with a class C core vacuole/endosome tethering (CORVET) complex and a seed plant-specific VAMP727-containing R-soluble NSF attachment protein receptor (SNARE) complex. These findings suggest that GPA5 functions as a plant-specific effector of GPA1/Rab5a and that it may mediate tethering and membrane fusion of DVs with PSVs through association with CORVET tethering complex and VAMP727-containing SNARE complex in the rice endosperm.

Next steps: Now, our major research focus is to decipher the detailed mechanisms by which GPA5 interacts with CORVET and SNARE complexes to mediate membrane fusion. In addition, as we rarely observed the typical glutelin-containing prevacuolar compartments (PVCs) in this study, we would also like to establish whether the GPA5-positive DVs define a special type of PVCs.

Yulong Ren, Yihua Wang, Tian Pan, Yunlong Wang, Yongfei Wang, Lu Gan, Zhongyan Wei, Fan Wang, Mingming Wu, Ruonan Jing, Jiachang Wang, Gexing Wan, Xiuhao Bao, Binglei Zhang, Pengcheng Zhang, Yu Zhang, Yi Ji, Cailin Lei, Xin Zhang, Zhijun Cheng, Qibing Lin, Shanshan Zhu, Zhichao Zhao, Jie Wang, Chuanyin Wu, Lijuan Qiu, Haiyang Wang, and Jianmin Wan. (2020). GPA5 Encodes a Rab5a Effector Required for Post-Golgi Trafficking of Rice Storage Proteins. Plant Cell; DOI: https://doi.org/10.1105/tpc.19.00863