We have two types of proteins – the ones that are found in humans, and those that were found in viruses. This means that they are different from each other in some ways. Viruses change a lot during their life cycles. The JUN protein is the viral form of the Jun protein that was originally identified as a transcription factor encoded by the Jun proto-oncogene.
It regulates gene expression and activates the AP-I promoter. In humans, JUN acts in the nucleus as a sequence-specific DNA-binding transcription factor. It activates transcription of various genes by binding to the GC box (CACCCTG).
JUN interacts directly with the basic region/leucine zipper (bZIP) domain of the Fos protein to form a ternary complex with DNA, Fos, and JUN. In this c-Jun protein complex, JUN is able to activate gene transcription by bringing the Fos protein to the GC box-containing promoters.
As a result of its activity, it regulates the expression of different genes. For example, it affects the expression of the human JunB gene. The JUN protein also acts as an intracellular transducer of growth factors, including epidermal growth factor (EGF), and of cellular stresses such as heat shock and oxidants.