A vacuolar sorting receptor-independent sorting mechanism for storage vacuoles in soybean seeds

Nobuyuki Maruyama,Hisakazu Hasegawa,Teruhiko Terakawa,Yuki Matsuoka,Masao Ishimoto,Kyoko Takagi,Tetsuya Yamada,Kazunori Yokoyama

doi:10.1038/s41598-017-18697-w

Abstract

The seed storage proteins of soybean (Glycine max) are composed mainly of glycinin (11S globulin) and β-conglycinin (7S globulin). The subunits of glycinin (A1aB1b, A1bB2, A2B1a, A3B4, and A5A4B3) are synthesized as a single polypeptide precursor. These precursors are assembled into trimers with a random combination of subunits in the endoplasmic reticulum, and are sorted to the protein storage vacuoles. Proteins destined for transport to protein storage vacuoles possess a vacuolar sorting determinant, and in this regard, the A1aB1b subunit contains a C-terminal peptide that is sufficient for its sorting to protein storage vacuoles. The A3B4 subunit, however, lacks a corresponding C-terminal sorting determinant. In this study, we found that, unlike the A1aB1b subunit, the A3B4 subunit does not bind to previously reported vacuolar sorting receptors. Despite this difference, we observed that the A3B4 subunit is sorted to protein storage vacuoles in a transgenic soybean line expressing the A3B4 subunit of glycinin. These results indicate that a protein storage vacuolar sorting mechanism that functions independently of the known vacuolar sorting receptors in seeds might be present in soybean seeds.

Highlights

Proteins destined for transport to protein storage vacuoles (PSVs) possess a vacuolar sorting determinant (VSD)[1]
The C-terminal 10 residues of the A1aB1b subunit of glycinin are sufficient as a C-terminal VSDs (ctVSDs) for sorting to the PSV, whereas the A3B4 subunit lacks a C-terminal peptide corresponding to the ctVSD of the A1aB1b subunit, and is assumed to contain a VSD other than the ctVSD type[9]
We examined the interaction between GmVSR(LU) and the VSDs of the A1aB1b subunit of glycinin

Summary

Introduction

Proteins destined for transport to protein storage vacuoles (PSVs) possess a vacuolar sorting determinant (VSD)[1]. Vacuolar sorting receptors (VSRs) for VSDs are involved in plant vacuolar sorting[1], among which are members of the BP-80 family of type I membrane proteins[10,11,12]. The C-terminal 10 residues of the A1aB1b subunit of glycinin are sufficient as a ctVSD for sorting to the PSV, whereas the A3B4 subunit lacks a C-terminal peptide corresponding to the ctVSD of the A1aB1b subunit, and is assumed to contain a VSD other than the ctVSD type[9]. Since endogenous glycinin forms hybrid molecules with expressed derivatives, it is difficult to examine sorting of the A3B4 subunit independently in transgenic soybean. The results indicate the existence of a vacuolar sorting mechanism in soybean seeds that functions independently of the known receptors in seeds

Methods

Results

Conclusion