Abstract
The transport mechanism of plasma membrane proteins to vacuoles has not been well characterized in plants. To clarify how to degrade plasma membrane proteins (PIP2a and LTI6b), we monitored the endocytotic pathway in transgenic Arabidopisis plants that express a fusion protein (GFP-PIP2a or GFP-LTI6b) and wild-type tobacco BY-2 cells. Because of the rapid delivery of endosomes to the vacuoles, endosomes were scarcely detectable. We found that E-64d, an inhibitor of papain family proteases, caused the accumulation of endosomes in sucrose-starved BY-2 cells. This result indicates that E-64d attenuate the fusion of endosomes to vacuoles. In transgenic Arabidopsis plants, treatment with E-64d induced the accumulation of GFP-fluorescent endosomes and inhibited the degradation of the fusion proteins. We found that two papain homologues (ENPs) are localized in accumulated endosomes. These results suggest that ENPs facilitate the fusion of endosomes with vacuoles at the final step of the endicytotic pathway.
