Transport of ricin and 2S albumin precursors to the storage vacuoles of Ricinus communis endosperm involves the Golgi and VSR-like receptors.

We have studied the transport of proricin and pro2S albumin to the protein storage vacuoles of developing castor bean (Ricinus communis L.) endosperm. Immunoelectron microscopy and cell fractionation reveal that both proteins travel through the Golgi apparatus and co-localize throughout their route to the storage vacuole. En route to the PSV, the proteins co-localize in large (>200 nm) vesicles, which are likely to represent developing storage vacuoles. We further show that the sequence-specific vacuolar sorting signals of both proricin and pro2SA bind in vitro to proteins that have high sequence similarity to members of the VSR/AtELP/BP-80 vacuolar sorting receptor family, generally associated with clathrin-mediated traffic to the lytic vacuole. The implications of these findings in relation to the current model for protein sorting to storage vacuoles are discussed.

Sequence-specific, Golgi-dependent vacuolar targeting of castor bean 2S albumin.

The targeting of the castor bean (Ricinus communis) 2S albumin precursor has been investigated by expressing cDNA in transformed tobacco (Nicotiana tabacum) leaf cells and by following biosynthesis in the native tissue. Correct targeting in both tissues was accompanied by processing of the precursor. Delivery to vacuoles was sensitive to brefeldin A (BFA) treatment in both tissues and to perturbation of COPII function in tobacco, supporting the view that transport through the Golgi is required. The targeting signal for this Golgi-dependent routing lies within the propeptide of the first heterodimer of proalbumin. This propeptide directed a normally secreted reporter protein to the vacuoles of tobacco cells in a Golgi-dependent manner in vivo, unless a critical Leu residue was mutated, supporting the view that a sequence-specific signal was needed to target a seed storage protein to the vacuoles of a vegetative cell.