Subcellular localization and targeting of glucocorticoid receptor protein fusions expressed in transgenic Arabidopsis thaliana

An animal system of inducible activation of protein fusions with the bindin g domain of glucocorticoid receptor (BDGR) was tested in Arabidopsis thalia na by monitoring dexamethasone (DEX)-induced nuclear targeting of reporter constructs. Two constructs containing green fluorescent protein (GFP), huma n homeobox protein Hanf-1 and Xenopus laevis BDGR were used, GFP/Hanf-1/BDG R and GFP/BDGR. The control construct contained GFP alone. In the absence o f DEX both fusion proteins were uniformly distributed in the cytoplasm of r oot cells, but showed strong association with plastids in plant aerial part s. DEX treatment of roots prompted a strong and reversible nuclear accumula tion of GFP/Hanf-1/BDGR, but not GFP/BDGR. Thus, in roots, the specific nuc lear translocation of GFP/Hanf-1/BDGR was driven by Hanf-1 and tightly regu lated by BDGR. However, in plant aerial parts treated with DEX, nuclear tra nslocation of GFP/Hanf-1/BDGR was observed only in a few cases, and most pa rt of the fusion protein was incorrectly and irreversibly targeted to plast ids. Protease X digestion of isolated chloroplasts showed that BDGR fusion proteins were translocated into the chloroplast envelope and bound to envel ope membranes, probably due to association with the chloroplast import appa ratus. Thus, for efficient use of the glucocorticoid-inducible system in pl ants, it will be necessary to modify BDGR structure to prevent incorrect ta rgeting of fusion proteins.