Biosynthesis of ribosome-inactivating proteins from callus and cell suspension cultures of Mirabilis expansa (Ruiz & Pavon)

Callus and cell suspension cultures from the little known Andean crop Mirab ilis expansa were developed and maintained on Murashige and Skoog medium su pplemented with 2,4-dichlorophenoxyacetic acid (1 mg/l) and kinetin (0.1 mg /l). Callus and cell suspension cultures were screened with antibodies rais ed against ME1 (27.5 kDa) and ME2 (27 kDa), two ribosome-inactivating prote ins (RIPs) previously found in roots of M. expansa. A 29-kDa protein found in callus and cell suspensions reacted strongly with ME1 antibodies. The 29 -kDa protein, named MEG, was purified to homogeneity by ammonium sulfate pr ecipitation and cation exchange perfusion chromatography. Amino acid N-term inal sequencing revealed close homology between MEC and ME1. The MEC amino acid sequence examined was highly conserved among RIPs from widely differen t sources. This new RIP was immunolocalized to the cell walls of callus and cell suspension cultures.