3.E.3. The HelioRhodopsin (HelioR) Family
Many organisms capture or sense sunlight using rhodopsin pigments, integral 7 TMS membrane proteins that bind retinal chromophores. Rhodopsin, both from animals and microorganims, are members of the TOG superfamily (Yee et al. 2013). The 7 helices form a pocket in which retinal is linked covalently as a protonated Schiff base to a lysine in the seventh TMS (Pushkarev et al. 2018). Heliorhodopsins are distantly related to microbial rhodopsins (MR, TC# 3.E.1), and animal rhodopsins (TC# 9.A. 14.1) but they are embedded in the membrane with their N termini facing the cell cytoplasm, an orientation that is opposite to that of MRs and animal rhodopsins. Heliorhodopsins show photocycles that last about 5 seconds, suggesting that they have light-sensory activity. The photocycles accompany retinal isomerization and proton transfer, as in other rhodopsins, but protons are never released from the protein, even transiently. Thus, they are not believed to transport protons across the membrane. Heliorhodopsins are abundant and distributed globally in Archaea, Bacteria, Eukarya and their viruses. They are widespread in the microbial world (Pushkarev et al. 2018).
The conserved E107 in TMS3 of heliorhodopsin 48C12 (TC# 3.E.3.1.1) may be the counterion of the protonated Schiff base, as in microbial rhodopsins. From pH titration studies, the pKa was determined to be 11.5 for the Schiff base. Light converts the all trans Schiff base retinal in heliorhodopsin to 13-cis. The Schiff base proton is transferred to the proton-accepting group in the M intermediate, which involves H23 and H80 but not E107. Thus, this accepting group is probably located within the N-terminal region of heliorhodopsins that faces the cytoplasmic side, assuming that the membrane topology of heliorhodopsins is opposite to that of microbial rhodopsins as proposed (Pushkarev et al. 2018).