JEM-1, a novel nuclear co-factor: localisation and functional interaction with AP-1

JEM-1 is a novel gene whose mRNA expression in acute promyelocytic leukemia (APL) is induced by retinoid treatments, The gene product, a 45 kDa basic nuclear factor containing a leucine repeat, was transiently expressed in He La or COS-7 cells and immunocharacterized within the nuclei in fine punctua ted structures which increase in size after cell transfection. Jem-1 was no t expressed in the nucleoli. Experimental deletion of peptide domains of Je m-1 (Jem Delta(331-400) and Jem Delta L179-206) showed that its C-terminal sequence (Thr(331)--> Leu(400)) is required for nuclear translocation, whil e the leucine repeat domain (Arg(179) --> Gru(206)) has no influence on sub cellular Localization. The Jem-1 protein was not detected in the PML-contai ning nuclear bodies or in speckled structures containing the splicing facto r SC-35, in contrast it was localized in the nucleus in structures containi ng activator protein-1 (AP-1). DNA mobility shift assays showed that the in vitro translated Jem protein interacts neither with the DNA binding site o f AP-1, nor directly with in vitro cc-translated c-Fos or/and c-Jun protein s bound to this specific sequence. interestingly, Jem-1-1 increased substan tially the transcriptional activity of c-Jun (three-fold) and more strongly that of ectopically co-expressed c-Fos and c-Jun (five- to six-fold), as m easured by a CAT reporter gene driven by a heterologous promoter containing the AP-1 binding site of the human collagenase gene. These synergistic eff ects were strongly Jem-1 dose-dependent. However, Jem-1 alone showed no act ivity on the collagenase promoter. A deletion of the Leucine repeat of Jem- 1 (Arg(179)- Glu(206)) did not diminish the enhancer capacity of Jem-1 on A P-1 activity. In contrast, the enhanced AP-I activity was abrogated when Je m-1 was deleted of its C-terminus (Thr(301) --> Leu(400)). We conclude that the 45 kDa nuclear product of the JEM-1 gene has features of a novel trans cription cofactor, which is enhancing AP-1 activity without directly intera cting with c-Jun or c-Fos proteins. Possible implications of these findings for APL cell maturation are discussed.