We therefore propose that JAB1 is a novel binding partner of Luman, which negatively regulates the activity of Luman by promoting its degradation. We found that overexpression of JAB1 shortened the half-life of Luman by 67%, and repressed its transactivation function on GAL4 and unfolded protein response element (UPRE)-containing promoters. More interestingly, the nuclear form of Luman was shown to promote the translocation of JAB1 into the nucleus. JAB1 also colocalized with Luman in transfected cells. Deletion mapping studies revealed that the MPN domain in JAB1 was essential and sufficient for the binding. We confirmed their direct binding by glutathione S-transferase pull-down assays, and verified the existence of such interaction in the cellular environment by mammalian two-hybrid and co-immunoprecipitation assays. In this study, we used the region of Luman containing the basic DNA-binding domain as bait in a yeast two-hybrid screen and identified the Jun activation domain-binding protein 1 (JAB1) or the COP9 signalosome complex unit 5 (CSN5) as an interacting protein. Luman/CREB3 (also called LZIP) is an endoplasmic reticulum (ER)-bound transcription factor that has been implicated in the ER stress response.
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