Ricin is a heterodimeric plant protein that is potently toxic to mammalian and many other eukaryotic cells. It is synthesized and stored in the endosperm cells of maturing Ricinus communis
seeds (castor beans). The ricin family has two major members, both, lectins, collectively known as Ricinus communis
agglutinin ll (ricin) and Ricinus communis
agglutinin l (RCA). These proteins are stored in vacuoles within the endosperm cells of mature Ricinus
seeds and they are rapidly broken down by hydrolysis during the early stages of post-germinative growth. Both ricin and RCA traffic within the plant cell from their site of synthesis to the storage vacuoles, and when they intoxicate mammalian cells they traffic from outside the cell to their site of action. In this review we will consider both of these trafficking routes.