SUBCELLULAR-LOCALIZATION OF THE 2S GLOBULIN NARBONIN IN SEEDS OF VICIA-NARBONENSIS

Narbonin is a 2S protein from the globulin fraction of narbon bean (Vi cia narbonensis L.) cotyledons. Its amino acid composition and the pat tern or its regulated accumulation in developing seeds led to the sugg estion that narbonin could be a storage protein. Therefore, it was exp ected to be present in protein bodies of the storage tissue cells. Com parison of the cDNA-derived amino acid sequence with a directly determ ined partial N-terminal sequence revealed that the primary translation product of narbonin mRNA lacks a transient N-terminal signal peptide (V.H. Nong et al., 1995, Plant Mol Biol 28: 61-72). Narbonin polypepti des that had been synthesized in a cell-free translation system supple mented with dog pancreas microsomes were not protected against degrada tion by posttranslationally added proteases (protease protection assay ). In accordance with the lack of a signal peptide this indicates that the polypeptide was not cotranslationally sequestered into the micros omes. The protein-body fraction that had been isolated from mature nar bon bean cotyledons by a non-aqueous gradient centrifugation procedure was free of narbonin, this was found in the soluble cell fraction. In electron micrographs, narbonin could be localized in the cytoplasm us ing the immune gold-labelling technique. Previously, it had already be en shown that narbonin is too slowly degraded during narbon bran germi nation to act as a storage protein. From all these results it has to b e concluded that narbonin is a cytoplasmic protein which does not belo ng to the storage proteins in the restricted sense. Other possible fun ctions are discussed.