| Description | JUNB has 347 amino acids with a predicted molecular weight 35,9 kD. Structurally, JUNB is similar to JUN, which contains a JNK docking site, nuclear localization signal, basic domain for DNA binding and a leucine zipper domain for dimerization. However, JUNB does not contain a JUNK phosphorylation site. Thus, the transactivation activity of JUNB is not regulated by JNK. |
| Function | JUNB is a member of JUN family (JUN, JUNB and JUND) that can dimerize with one another, or with members of Fos and ATF families, to form AP-1 transcription factor. Comparing with JUN, the transactivation activity of JUNB is much weaker. Due to the small differences on the amino acid sequences in the basic DNA bindind domain, and leucine zipper domain, JUNB requires multiple AP-1 DNA binding sites for sufficient DNA binding. A number of studies demonstrated that JUNB antagonizes the functions of JUN in cell cycle regulation, proliferation and transformation by competing with JUN to form less efficient transactivating dimers. Thus, JUNB was considered as a tumor suppressor.
In gene knockout studies, mice lacking Jun gene die during embryonic day 12.5 and 13.5, whereas embryos lacking JunB die earlier, around day 9.5, owing to vascular defects in the placenta and extraembryonic tissue. Interestingly; gene knock-in experiment indicated that JUNB could partially substitute the activities of JUN in mouse development and cell proliferation. As possible explanation for this is that in presence of JUN, JUNB is a negative regulator for JUN. In contrast, in the absence of JUN, JUNB may substitute JUN and activate AP-1 target genes required for development and cell proliferation. |
