Cation Binding to Xanthorhodopsin: Electron Paramagnetic Resonance and Magnetic Studies

Zinc Binding to Heliorhodopsin

Heliorhodopsin (HeR), a recently discovered new rhodopsin family, has an inverted membrane topology compared to animal and microbial rhodopsins, and no ion-transport activity. The slow photocycle of HeRs suggests a light-sensor function, although the function remains unknown. HeRs exhibit no specific binding of monovalent cations or anions. Despite this, ATR-FTIR spectroscopy in the present study demonstrates binding of Zn²⁺ to HeR from Thermoplasmatales archaeon (TaHeR). The biding of Zn²⁺ to 0.2 mM K_d is accompanied by helical structural perturbations without altering its color. Even though ion-specific FTIR spectra were observed for many divalent cations, only helical structural perturbations were observed for Zn²⁺-binding. Similar results were obtained for HeR 48C12. These findings suggest a possible modification of HeR function by Zn²⁺.