Pathways for protein transport to seed storage vacuoles

Plant vacuoles have multiple functions: they can act both as digestive organelles and as receptacles for storage proteins. Different types of vacuoles can coexist in the same cell, which adds complexity to the process of targeting to these compartments. A fuller understanding of this process is of evident value when endeavouring to exploit the plant secretory pathway for heterologous protein production. Positive sorting signals are required in order to sort proteins to vacuoles, and these have been split into three groups: ctVSS [C-terminal VSS (vacuolar sorting signals)], ssVSS (sequence-specific VSS) and physical structure VSS. The current working model posits that soluble proteins are delivered from the Golgi apparatus to the lytic vacuoles in clathrin-coated vesicles by virtue of their ssVSS, or to the storage vacuole [PSV (protein-storage vacuole)] in dense vesicles in a manner dependent on ctVSS or physical structure VSS. Although targeting to LV appears to be receptor-mediated, no such receptor has been identified for the recruitment of proteins to the PSV. We have studied the vacuolar targeting of two castor bean (Ricinus communis L.) storage proteins, proricin and pro 2 S albumin, in their native endosperm and in the heterologous system of tobacco protoplasts. We have found that both these proteins contain bona fide ssVSS and bind to sorting receptors in vitro in a similarly sequence-specific manner. The apparent similarities to lytic VSS and possible implications with respect to the working model for transport to storage vacuoles are discussed.