Different binding modalities of quercetin to inositol-requiring enzyme 1 of S. cerevisiae and human lead to opposite regulation

The flavonoid Quercetin (Qe) was identified as an activator of Inositol-requiring enzyme 1 (IRE1) in S. cerevisiae ( sc Ire1p), but its impact on human IRE1 ( h IRE1) remains controversial due to the absence of a conserved Qe binding site. We have explored the binding modes and effect of Qe on both sc Ire1p and h IRE1 dimers using in silico and in vitro approaches. The activation site in sc Ire1p stably accommodates both Qe and its derivative Quercitrin (Qi), thus enhancing the stability of the RNase pocket. However, the corresponding region in h IRE1 does not bind any of the two molecules. Instead, we show that both Qe and Qi block the RNase activity of h IRE1 in vitro, with sub-micromolar IC 50 values. Our results provide a rationale for why Qe is an activator in sc Ire1p but a potent inhibitor in h IRE1. The identification of a new allosteric site in h IRE1 opens a promising window for drug development and UPR modulation.