Involved in tetrahydrobiopterin biosynthesis (By similarity). Interacts with and acts downstream of the E3 ubiquitin-protein ligase SDIR1 in abscisic acid (ABA) and salt stress signaling. Regulates the expression of the bZIP transcription factor ABI5, which mediates responses to ABA during seed germination and salt stress. The SDIR1-ATP1/SDIRIP1 complex plays an important role in ABA signaling through the ubiquitination pathway (PubMed:25616872). Acts downstream of AIRP2 in regulation of ABA signaling during drought stress (PubMed:28626006). (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin = (6R)-L-erythro-6,7-dihydrobiopterin + H2O Interacts with SDIR1 (PubMed:25616872). Interacts with AIRP2 (PubMed:28626006). Ubiquitinated by SDIR1. Ubiquitination leads to its subsequent degradation, thus controlling abscisic acid (ABA) signaling (PubMed:25616872). Ubiquitinated by AIRP2. Ubiquitination leads to its subsequent degradation, thus controlling abscisic acid (ABA) signaling during drought stress (PubMed:25616872). Belongs to the pterin-4-alpha-carbinolamine dehydratase family.