A C-terminal sequence of soybean beta-conglycinin alpha' subunit acts as a vacuolar sorting determinant in seed cells

In maturing seed cells, many newly synthesized proteins are transported to the protein storage vacuoles (PSVs) via vesicles unique to seed cells. Vacuolar sorting determinants (VSDs) in most of these proteins have been determined using leaf, root or suspension-cultured cells apart from seed cells. In this study, we examined the VSD of the alpha' subunit of beta-conglycinin (7S globulin), one of the major seed storage proteins of soybean, using Arabidopsis and soybean seeds. The wild-type alpha' was transported to the matrix of the PSVs in seed cells of transgenic Arabidopsis, and it formed crystalloid-like structures. Some of the wild-type alpha' was also transported to the translucent compartments (TLCs) in the PSV presumed to be the globoid compartments. However, a derivative lacking the C-terminal 10 amino acids was not transported to the PSV matrix, and was secreted out of the cells, although a portion was also transported to the TLCs. The C-terminal region of alpha' was sufficient to transport a green fluorescent protein (GFP) to the PSV matrix. These indicate that alpha' contains two VSDs: one is present in the C-terminal 10 amino acids and is for the PSV matrix; and the other is for the TLC (the globoid compartment). We further verified that the C-terminal 10 amino acids were sufficient to transport GFP to the PSV matrix in soybean seed cells by using a transient expression system.