Prosomes (proteasomes) of higher plants

Abstract

From different plant tissues such as tobacco (Nicotiana rustica), potato (Solanum tuberosum), and mung bun (Phaseolus radiatus), ring- or cylinder-shaped particles called prosomes wrere isolated by either sucrose gradient centrifugation or fast protein liquid chromatography (FPLC). These particles have a diameter of 12 to 14 nm and a length of 16 to 18 nm. They migrate under conditions of nondenaturing gel electrophoresis as one distinct band. Sedimentation coefficient and buoyant density in Cs 2SO 4. of the plant prosomes were determined by analytical ultracentrifugation to be approximately 23S and 1.23g/cm 3, respectively. The total molecular mass was estimated by gel filtration to be 650 kDa. Plant prosomes are composed of 12 to 15 proteins with molecular masses in the range of 24 to 35 kDa with isoelectric points of pH 5 to 7 as revealed by twodimensional gel electrophoresis. The protein patterns or prosomes from the three different plant species are very similar. Polyclonal antisera against potato prosomes reacted in Western blots with prosomal proteins of all three plant species. They also bind to some prosomal proteins of animal species. Antisera against animal prosomes react with some proteins of plant prosomes. As shown by lectin blotting, plant prosomes .are glycosylated carrying glucosyl- or mannosyl, and N-acetylgalactosaminyl residues. Prosomal preparations contain non-stoichiometric amounts of small RNA of about 80 kDa. These results suggest that plant prosomes are structurally and functionally homologous to prosomes of other eukaryotic cells.