Decoding Lysine-11 Signals in Ubiquitination
The diverse outcomes of ubiquitination primarily relate to the flexibility of ubiquitin in forming homo- or heterotypic chains on each of its seven lysine residues which in turn stimulate distinct downstream signaling pathways. These ubiquitin signals must be selectively initiated on the substrate protein and subsequently decoded to facilitate the desired cellular function. These initiation and decoding steps often involve additional post-translational modifications and ubiquitin receptor proteins, but the enzymes and ubiquitin chains involved for many ubiquitinated substrates are not clear. Here, I have explored the initiation and decoding of ubiquitin signals, focusing on lysine-11 (K11) linked polyubiquitin chains and their role in protein degradation. I established in vitro assays to understand how K11-chains are decoded and whether these chains act as a signal for proteasome-mediated degradation. Pure homotypic K11-chains did not bind the proteasome or its associated ubiquitin binding proteins, but did bind to the mitophagy ubiquitin receptors, MyosinVI and TAX1BP1. Heterotypic K11/K48 linkages not only bound the proteasome but also stimulated degradation of the cell cycle substrate, cyclin B1. To further explore the functions of K11-chains I focused on the hypoxia inducible transcription factor (HIF) pathway, as K11-ubiquitination had been implicated in proteasome-independent degradation of the transcription factor. I established an in vitro assay to initiate HIF ubiquitination, via prolyl hydroxylation, and determine the type of ubiquitin chains involved. Recombinant HIF isoforms were rapidly hydroxylated when incubated with cell extracts. Moreover, the levels of iron and small molecule metabolites within the lysates regulated HIF hydroxylation. However, this hydroxylation was insufficient to reproducibly promote HIF ubiquitination or determine the ubiquitin chains involved. While the nature of the polyubiquitin chains formed in the HIF pathway remain elusive, my studies identify distinct roles for homotypic and heterotypic K11-polyubiquitination in proteasome-mediated degradation.