The CCAAT-binding complex mediates azole susceptibility of Aspergillus
fumigatus by suppressing SrbA expression and cleavage
Abstract
In fungal pathogens, the transcription factor SrbA (a sterol regulatory
element-binding protein, SREBP) and CBC (CCAAT binding complex) have
been reported to regulate azole resistance by competitively binding the
TR34 region (34 mer) in the promoter of the drug target gene, erg11A.
However, current knowledge about how the SrbA and CBC coordinately
mediate erg11A expression remains limited. In this study, we uncovered a
novel relationship between HapB (a subunit of CBC) and SrbA in which
deletion of hapB significantly prolongs the nuclear retention of SrbA by
increasing its expression and cleavage under azole treatment conditions,
thereby enhancing Erg11A expression for drug resistance. Furthermore, we
verified that loss of HapB significantly induces the expression of the
rhomboid protease RbdB, Dsc ubiquitin E3 ligase complex, and signal
peptide peptidase SppA, which are required for the cleavage of SrbA,
suggesting that HapB acts as a repressor for these genes which
contribute to the activation of SrbA by proteolytic cleavage. Together,
our study reveals that CBC functions not only to compete with SrbA for
binding to erg11A promoter region but also to affect SrbA expression,
cleavage, and translocation to nuclei for the function, which ultimately
regulate Erg11A expression and azole resistance.