Dear Editor,
Heliorhodopsins (HeRs) are a new class of Microbial rhodopsins (MRs),with a low homology (<15%) to all other MRs and a unique membrane topology in which their N-termini face the intracellular side and C-termini face the extracellular side.1 To explore the photoactivation mechanism of HeRs,we sought to solve the structure of HeR 48C12 (hereafter referred to as HeR).HeR protein was expressed in insect cells and purified by standard membrane purification methods (Supplementary information,Fig.S1a and Data S1).