The small GTPase Rab5a and its guanine nucleotide exchange factors are involved in post-Golgi trafficking of storage proteins in developing soybean cotyledon

Storage protein is the most abundant nutritional component in soybean seed. Morphology-based evidence has verified that storage proteins are initially synthesized on the endoplasmic reticulum, and then follow the Golgi-mediated pathway to the protein storage vacuole. However, the molecular mechanisms of storage protein trafficking in soybean remain unknown. Here, we clone the soybean homologs of Rab5 and its guanine nucleotide exchange factor (GEF) VPS9. GEF activity combined with yeast two-hybrid assays demonstrated that GmVPS9a2 might specifically act as the GEF of the canonical Rab5, while GmVPS9b functions as a common activator for all Rab5s. Subcellular localization experiments showed that GmRab5a was dually localized to the trans-Golgi network and pre-vacuolar compartments in developing soybean cotyledon cells. Expression of a dominant negative variant of Rab5a, or RNAi of either Rab5a or GmVPS9s, significantly disrupted trafficking of mRFP-CT10, a cargo marker for storage protein sorting, to protein storage vacuoles in maturing soybean cotyledons. Together, our results systematically revealed the important role of GmRab5a and its GEFs in storage protein trafficking, and verified the transient expression system as an efficient approach for elucidating storage protein trafficking mechanisms in seed.