4 PUTATIVE, GLYOXYSOME MEMBRANE-PROTEINS ARE INSTEAD IMMUNOLOGICALLY-RELATED PROTEIN BODY MEMBRANE-PROTEINS

Previous research revealed that peroxisomes/glyoxysomes in plants, mam mals, and yeasts possessed a prominent, integral peroxisome membrane p rotein (PMP) with a molecular mass in the range 21-26 kDa. We also fou nd a major, low molecular mass polypeptide in the glyoxysome membrane fractions from four oilseed species: putative PMP26 in cotton (Gossypi um hirsutum L.), PMP22 in cucumber (Cucumis sativus L.), PMP24.5 in su nflower (Helianthus annuus L.), and PMP24 in castor bean (Ricinus comm unis L.). Rabbit antiserum, produced against all the proteins which we re solubilized from membranes recovered from isolated cotton glyoxysom e fractions, recognized these four putative PMPs on Western blots as w ell as similar molecular mass polypeptides in mitochondria and protein bodies isolated from cotton and cucumber cotyledons. Postembedding, i mmunogold analyses of cotton and cucumber cotyledons, however, reveale d that antibodies which were affinity-purified to the putative cotton glyoxysome 26-kDa polypeptide specifically bound to membranes of prote in bodies, but not to membranes of glyoxysomes nor mitochondria. Antib odies to the bean tonoplast intrinsic protein (alpha-TIP), a known pro tein body membrane protein (PBMP), also recognized the four putative P MPs on Western blots and co-localized with anti-cotton PMP26 IgGs on t hin sections to membranes of protein bodies. Intact protein bodies wer e not the source of the PBMPs in the oilseed organelle fractions; immu nogold labeling revealed immunoreactive, small indiscrete vesicles int erspersed among glyoxysomes and mitochondria in gradient fractions. Th us, putative prominent PMPs believed previously by us and others to be important in biogenesis and/or function of plant peroxisomes were dis covered to be antigenically-related PBMPs of similar low molecular mas s whose function in oilseeds has yet to be elucidated.