Figure 7.

Model for VSR_PS-1 Function in Plants.

The trans-Golgi (left) is the region of the secretory pathway where sorting of vacuolar soluble proteins is believed to occur. At this neutral pH, the plant vacuolar sorting receptor (brackets) sorts (step 1) proaleurain (gray squares) from other soluble proteins (white squares). The detail shows more precisely the structure of VSR_PS-1, with its N-terminal domain (N) interacting with the ssVSD carried by proaleurain (aleu.). It also shows the C-terminal domain (C) of VSR_PS-1 on the cytosolic side of the membrane that is available for contacts with proteins such as adaptins that are involved in CCV formation. The complex of VSR_PS-1 and proaleurain (step 2) is packed into transport vesicles and targeted to an acidic compartment (right). The decrease in pH causes the ligand to be released from VSR_PS-1 (step 3) within the acidic compartment, which is possibly a prevacuole. The free receptor then recycles back to the Golgi (step 4), again by means of transport vesicles. We know that CCVs are used by the receptor in plants to shuttle between the Golgi and an acidic compartment, but we do not know in which direction of transport. This model is based on our demonstration of VSR_PS-1 function in yeast and on previously published characteristics (Kirsch et al., 1994; Paris et al., 1997).