Localization of β-glucuronidase in protein bodies of transgenic tobacco seed by fusion to an amino terminal sequence of the soybean lectin gene

We examined the effect of the first 32 amino acids of the soybean seed lectin gene on subcellular localization of β-glucuronidase (GUS) in transgenic tobacco seeds. The coding region of a non-glycoslyated GUS protein served as a reporter gene inserted between two expression cassettes containing the 5' promoter and 3' non-coding regions of the lectin gene. These expression cassettes were identical except for the presence or absence of the 32 amino acid N-terminal sequence that precedes the mature lectin protein. Tobacco leaf disks were transformed by Agrobacterium tumefaciens and 13 independently transformed lines were tested for genetic segregation ratios to define the number of independently segregating insertion events. Both promoters resulted in developmental and tissue specific expression of the GUS reporter gene. We determined that when the GUS protein is preceded by the 32 amino acid N-terminal lectin peptide, there is significant association of GUS activity and protein with the protein bodies as determined by subcellular fractionation and by localization using immunogold electron microscopy. Although unexpected, these results indicate that the 32 amino acid N-terminal sequence of the lectin protein which serves as a signal sequence is sufficient to send the attached protein to the protein bodies. These expression cassettes with and without the 32 amino acid N-terminal sequence should be useful for expression of foreign proteins either in the protein bodies or in the cytosol during seed development of soybean or other plants.