The ubiquitylation of target proteins is one of the most important post-translational modifications and plays essential roles in numerous cellular processes. Ubiquithylation is performed by a sequential enzyme cascade of E1 activating enzymes, E2 conjugating enzymes and E3 binding enzymes. For many years it was thought that UBA1 was the only E1 enzyme that activates ubiquitin, until a second enzyme that activates ubiquitin was discovered in 2007: UBA6.
A potential target for drug development
UBA6 is present only in vertebrates and sea urchins. Curiously, UBA6 is an unusual E1 enzyme in that it activates both ubiquitin and the ubiquitin-like protein (Ubl) FAT10. Due to its narrow range of ubiquitylation events, compared to the action of the generic ubiquitin activating enzyme UBA1, and being the only FAT10 bond that catalyzes E1, UBA6 is considered a potential drug target. To further explore the targeted inhibition of UAB6, it is essential to understand its dual specificity and to identify variants of the enzyme that are altered in the activation of ubiquitin or FAT10.
The research group of Prof Hermann Schindelin at the Rudolf Virchow Center of the University of Würzburg reports the first structures of UBA6, in complex with ATP or FAT10. Interestingly, their structural and modeling studies also revealed how UBA6 allows for dual recognition of ubiquitin and FAT10. Another key discovery is the identification of UBA6 variants that selectively abolish the activation of ubiquitin or FAT10. “These results provide the basis for studying the individual roles that UBA6 is playing in the activation of ubiquitin or FAT10 in downstream cellular pathways,” says Schindelin.
Entangled in various diseases
As ubiquitylation and FAT10ylation are involved in multiple cellular processes, it is not surprising that malfunctions in one or more components of this system lead to a variety of diseases. UBA6-mediated proteasome degradation has been reported to be involved in the physiological and pathophysiological states associated with the brain in mice. Interestingly, UBA6 was overexpressed in the human brain of Alzheimer’s disease patients. The tumor suppressor protein p53 is a FAT10 substrate and a double negative regulation of FAT10 and p53 has been observed to be critical in the control of tumorigenesis, which is consistent with the overexpression of FAT10 in many types of cancer cells.
While predictions about the orientation of ubiquitin in complex with UBA6 could be readily generated based on the structure of the UBA6-ATP complex and the available UBA1-Ub structures, it seemed impossible to predict how FAT10 and, in particular, its N-terminal domain. (NTD) would interact with UBA6. Hence, the determination of the UBA6-FAT10 co-crystal structure was a crucial step. Based on the experimentally derived structures and the UBA6-ubiquitin model complex, a selectivity switch was identified which led to the subsequent discovery of UBA6 variants that selectively abolish the activation of both modifiers.
Future studies on UBA6 with selectively altered mutants are needed to investigate possible links between UBA6-catalyzed ubiquitylation and FAT10 hilation in the context of cancer. “If causal relationships can be established, the targeted inhibition of UBA6 by enzymes and siliconeWe will conduct screening approaches based on “, explains Schindelin.” With the set of mutans that selectively alter ubiquitylation or FAT10ylation we will study the selective impairment of both processes in cellular experiments with particular attention to cancer cells. “
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