ABSTRACT
Engrailed homeoprotein, a transcription factor involved in midbrain/hindbrain patterning, primarily localizes to the cell nucleus. However, significant amounts of the protein are also found in the cell cytoplasm or associated with membrane microdomains enriched in cholesterol and glycosphingoglycolipids (Joliot, A., Trembleau, A., Raposo, G., Calvet, S., Volovitch, M. and Prochiantz, A. (1997) Development 124, 1865-1875). This non-nuclear localization, observed in vitro and in vivo, led us to investigate the possibility that Engrailed be transferred between nuclear and non-nuclear compartments. Monkey COS-7 cells expressing chick Engrailed-2 (cEN2) were fused with 3T3 mouse fibroblasts and the passage of cEN2 from COS-7 to 3T3 nuclei was followed in the interspecies heterokaryons. We find that, 10 minutes following cell fusion, cEN2 is detected in the 3T3 nuclei of 80% of the heterokaryons demonstrating rapid cEN2 nuclear export. Export from donor nuclei can be saturated and is strongly reduced after deletion of a 11 amino acid-long Δ1 sequence present within a slightly larger domain that extends between helices 2 and 3 of the homeodomain and shows strong similarities with leucine-rich nuclear export signals (NES). This putative NES, when fused with a nuclear reporter protein, allows its nuclear export, demonstrating that it is not only necessary but also sufficient for nuclear export and can therefore be considered as a true nuclear export sequence. In an earlier report (Joliot, A., Maizel, A., Rosenberg, D., Trembleau, A., Dupas, S., Volovitch, M. and Prochiantz, A. (1998) Current Biology 8, 856-863), we demonstrated that the Δ1 sequence is necessary for the access of cEN2 to the lumen of a membrane compartment and for its intercellular transfer. The present study thus strongly suggests that the regulation of Engrailed nuclear export could play a role not only in Engrailed transcriptional activity but also in its ability to gain access to a secretory compartment.
