Subcellular location and expression level of a chimeric protein consisting of the maize waxy transit peptide and the beta-glucuronidase of Escherichia coli in transgenic potato plants

The transit peptide of the maize waxy protein (a nuclear-encoded amyloplast protein of the maize endosperm) was studied with respect to its role in subcellular protein targeting in transgenic potato plants. TP30, a chimeric precursor protein consisting of the waxy transit peptide and an additional 34 amino acids of the mature waxy protein fused to the beta-glucuronidase of Escherichia coli, was expressed in potato plants under the control of the 35S promoter of cauliflower mosaic virus. This fusion protein is imported not only into amyloplasts, the natural target organelles in the maize plant, but also into chloroplasts. In contrast, Gus, the beta-glucuronidase alone, which was also expressed in parallel experiments in transgenic potato plants is always found in the cytosol of the plant cells. As a consequence of the different subcellular locations of TP30 and Gus, we observed differences in the expression rates of the respective proteins in leaf cells, resulting in higher steady state levels of TP30 compared to Gus. In tuber cells, no correlation between intracellular location and expression of the proteins was found.