Translational regulation of the JunD messenger RNA.

JunD, a member of the Jun family of nuclear transcription proteins, dimerizes with Fos family members or other Jun proteins (c-Jun of JunB) to form the activator protein 1 (AP-1) transcription factor. The junD gene contains no introns and generates a single mRNA. Here we show that two predominant JunD isoforms are generated by alternative initiation of translation, a 39-kDa full-length JunD protein (JunD-FL) by initiation at the first AUG condon downstream of the mRNA 5' cap and a shorter, 34-kDa JunD protein (PiJunD) by initiation at a second in-frame AUG condon. The JunD mRNA contains a long, G/C-rich 5'-untranslated region that is predicted to be highly structed and is important for regulating the ratio of JunD-FL and PiJunD protein expression. A third functional out-of-frame AUG directs translation from a short open reading frame positioned between the JunD-FL and PiJunD start sites. In addition, three non-AUG codons also support translation, an ACG condon (in frame with JunD) and a CUG are positioned in the 5'-untranslated region, and a CUG codon (also in-frame with JunD) is located downstream of the short open reading frame. Mutation of these start sites individually had no affect on PiJunD protein levels, but mutation of multiple upstream statr sites led to an increase in PiJunD protein levels, indicating that these condons can function cumulatively to suppress PiJunD translation. Finally, we show that the JunD mRNA does not possess an internal ribosome entry site and is translated in a cap-dependent manner.