The Synthesis of Ricinus communis Lectins
Ricinus communis agglutinin II (ricin) and R. communis agglutinin I are cytotoxic seed lectins whose study has contributed to our understanding of precursor synthesis, membrane translocation, ribosome inactivation, intracellular trafficking, vacuolar targ
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Abstract Ricinus communis agglutinin II (ricin) and R. communis agglutinin I are cytotoxic seed lectins whose study has contributed to our understanding of precursor synthesis, membrane translocation, ribosome inactivation, intracellular trafficking, vacuolar targeting, protein assembly and quality control in plant cells. This chapter will focus largely on the targeting, quality control and protein assembly of these two closely related sugar binding, ribotoxic proteins.
1 Introduction The two lectins from the seeds of Ricinus communis (castor oil plant) are carbohydrate-binding proteins, each possessing at least two sugar-binding sites. As such they can agglutinate cells, as was first observed over a century ago by Stillmark (1889) who was studying the toxic effects of castor bean extracts on blood. The toxic agglutinating factor he discovered was a protein he termed ricin. Much later, it was shown that the toxic and agglutinating properties reside in two distinct but closely related proteins: ricin (known alternatively as R. communis agglutinin II; RCA II), which is a relatively weak haemagglutinin but a very potent cytotoxin, and R. communis agglutinin I (RCA I), which is a strong haemagglutinin showing weak cytotoxicity (Nicolson et al. 1974; Olsnes et al. 1974). Since these proteins were derived from plants, they came to be known as phytohaemagglutinins, but were later classified as 7S globulins, vicilin-like lectins, and as type II ribosome-inactivating proteins (RIPs). Besides providing the blueprint for the biosynthesis and cell biology of type II RIPs, the study of the synthesis of ricin and RCA has shed light on several
L. Frigerio (*) and L.M. Roberts Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK e-mail: [email protected]
J.M. Lord and M.R. Hartley (eds.), Toxic Plant Proteins, Plant Cell Monographs 18, DOI 10.1007/978-3-642-12176-0_10, # Springer-Verlag Berlin Heidelberg 2010
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important cellular processes such as translocation into the endoplasmic reticulum (ER), ER quality control and protein sorting to storage vacuoles. In particular, the study of ricin A chain provided the first indication for the presence of an operational ER-associated degradation (ERAD) machinery in plant cells (Di Cola et al. 2001). The study of vacuolar sorting of proricin challenged the “multivacuole hypothesis” and contributed to a revision of the current model for sorting to such organelles (Frigerio et al. 2008). The history, cell biology, toxicity and potential uses of ricin have been reviewed extensively (Lord et al. 1994; Frigerio and Roberts 1998; Olsnes 2004; Audi et al. 2005). In this chapter, we focus on recent advances in the understanding of the synthesis and intracellular fate of ricin, RCA and their individual sub-units.
2 Ricin 2.1
Synthesis and Quality Control of Proricin
Ricin is the best known of the castor bean lectins, primarily because it is the archetypal member of the A–B family of plant and bacterial proteins that
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