A scaffold protein that chaperones a cysteine-sulfenic acid in HO signaling.

In Saccharomyces cerevisiae, Yap1 regulates an HO-inducible transcriptional response that controls cellular HO homeostasis. HO activates Yap1 by oxidation through the intermediary of the thiol peroxidase Orp1. Upon reacting with HO, Orp1 catalytic cysteine oxidizes to a sulfenic acid, which then engages into either an intermolecular disulfide with Yap1, leading to Yap1 activation, or an intramolecular disulfide that commits the enzyme into its peroxidatic cycle.