Ultrafast Dynamics of Heliorhodopsins

Heliorhodopsins (HeR) constitute a new rhodopsin family and show only <15% sequence identities with type-1 and type-2 rhodopsins. The large difference in amino acid sequence between HeRs and other rhodopsins raises a question whether their biological function is triggered by efficient and rapid photoisomerization of the retinal chromophore as in the case of other rhodopsins. We performed femtosecond time-resolved absorption measurements of two HeRs, HeR 48C12 and HeR from Thermoplasmatales archaeon SG8-52-1. Both HeRs exhibit excited-state absorption around 480 nm and stimulated emission in the >650 nm region, and these transient signals decay concomitantly with appearance of photoproduct absorption on a subpicosecond time scale. The observed spectral change indicates that ultrafast retinal photoisomerization proceeds in the femtosecond time region. The transient spectra and dynamics of HeRs are surprisingly similar to those of type-1 rhodopsins, despite remarkable differences in amino acid arrangement in the hydrophobic region of the retinal binding site.