Abstract
Bdelloid rotifers are renowned for their extraordinary resilience. They can survive desiccation and long-term freezing and revive once favorable conditions return. Such remarkable survival abilities suggest that the evolution of specialized molecular adaptations sustain pH homeostasis under extreme environmental stress. In the present study, we identify and functionally characterize an isoform of the voltage-gated proton channel (Hv) from the bdelloid rotifer Adineta ricciae, which we name ArHv2. Unlike its paralog ArHv1 and human Hv1 that activate only at positive membrane potentials and contribute modestly to proton extrusion under normal conditions, the ArHv2 activates even at negative potentials, enabling rapid and efficient proton efflux or influx when cells face freezing or metabolic arrest. We discover that this behavior is conferred by a positively charged lysine located in the S4 transmembrane segment. This lysine is absent from other known Hv homologs but is highly conserved in bdelloid rotifer Hv2 channels. We further delineated the molecular mechanism by which this lysine fine-tunes ArHv2 proton channel activation. The discoveries in the present work highlight how the evolution of ion channel architecture at the amino acid level can yield physiological adaptations.