Type I Ribosome-Inactivating Proteins from Saponaria officinalis
Saporins are ribosome-inactivating proteins (RIPs) extracted from different tissues of the soapwort plant (Saponaria officinalis L.). While the biosynthesis of these proteins and their roles in planta have received little attention, saporins have been ext
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Abstract Saporins are ribosome-inactivating proteins (RIPs) extracted from different tissues of the soapwort plant (Saponaria officinalis L.). While the biosynthesis of these proteins and their roles in planta have received little attention, saporins have been extensively used for the production of targeted toxins for therapeutical and research applications. The biochemical features of one group of closely related saporin isoforms, collectively named SO6, have been characterized in considerable detail. In this chapter, we summarize available information on the saporin family of proteins, including their catalytic activity, 3D-structure, and biosynthetic and intoxication pathway(s), emphasizing the differences between the different family members and the characteristics that distinguish saporin from the catalytic subunit of the prototype Type II RIP ricin. The use of heterologous systems for the production of saporin and saporin-based chimeric toxins is also described.
1 Introduction Ribosome-inactivating proteins (RIPs) are potent inhibitors of protein synthesis that act by catalytically depurinating an adenine residue (A4324 in rat) present in a conserved stem-loop region of 23/26/28S ribosomal RNA (rRNA), causing an irreversible arrest in protein synthesis (Endo and Tsurugi 1987, 1988; Endo et al. 1988; Hartley et al. 1991). The prototype RIP is the ricin AB dimer, whose A. Lombardi, R.S. Marshall, M.S. Fabbrini, and A. Ceriotti (*) Istituto di Biologia e Biotecnologia Agraria, Consiglio Nazionale delle Ricerche, Via Bassini 15, 20133 Milano, Italy e-mail: [email protected]; [email protected]; [email protected]; [email protected] C. Savino Istituto di Biologia e Patologia Molecolari, Consiglio Nazionale delle Ricerche, c/o Universita` “Sapienza” di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy 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_4, # Springer-Verlag Berlin Heidelberg 2010
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biochemical features, catalytic activity, biosynthetic pathway, and intracellular transport have been studied in great detail. Ricin is synthesized as an inactive single-sized precursor that is transported to the protein storage vacuoles of castor bean endosperm cells and processed into disulphide-linked A (RTA) and B (RTB) chains (Butterworth and Lord 1983; Hiraiwa et al. 1997). The mechanism by which the ricin dimer intoxicates mammalian cells has also been thoroughly characterized (see chapter, “How Ricin Reaches its Target in the Cytosol of Mammalian Cells” by Spooner et al. in this volume) (Sandvig and van Deurs 2000; Lord et al. 2003). In contrast, little is known about the biosynthesis and trafficking of Type I (single chain) RIPs, their physiological function(s) in planta, and the mechanism(s) by which they reach the cytosol after uptake by mammalian cells. The name saporin collectively identifies a family of RIPs that accumulate in different soapwort (Saponaria officinalis L.) tissue
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