Rhodopsin-cyclases for photocontrol of cGMP/cAMP signaling

The recently discovered photoreceptor family of enzyme rhodopsins comprises a number of Rhodopsin-cyclases (RhGC/ACs) and Rhodopsin-phosphodiesterases (RhPDEs). These are rhodopsins that are either directly connected to an enzyme as realized in fungal class of Blasticladiomycetes (Avelar et al. 2014) or the coupling is indirect via a His-kinase and a Response Regulator as in case of the Histidine Kinase Rhodopsins of Chlorophyte algae (Luck et al. 2012).

We characterized two recombinant Rhodopsin-cyclases from Blastocladialla emersonii (BeRhGC) (Scheib et al. 2015) and Catenaria anguillulae (CaRhGC) (Scheib et al. 2018) and have shown that both enzymes are completely inactive in the dark and are active upon illumination. The enzymatically active state is formed within 10 to 40 ms after a flash and decays within 100 to 500 ms depending on the species. The cyclases are highly selective for GTP whereas ATP is acting as a competitive inhibitor. Coexpression of RhGC with a cGMP-channel from olfactory neurons and other sources in Xenopus oocytes , ND cells, HEK cells or neurons generating a large photocurrent upon illumination and conferring a high light sensitivity to the cell of interest. We are currently working on a cGMP-sensitive potassium channel in order to use our Two-Component Approach for rhodopsin-based hyper­pola­ri­zation in the neurosciences and cell biology.

In parallel we work on Chlamydomonas Histidine kinase rhodopsins (HKRs) from Chlamy­domo­nas reinhardtii (Luck and Hegemann 2017; Luck et al. 2012)and Ostreococcus tauri (Scheib unpublished). Both are synthesized in the cell as blue-green sensitive rhodopsins but are converted upon illumination into a UVA-sensitive species (Rh-UV) with maximal absorption near 380 nm. The thermal stability of the Rh-UV is very high but varies between many minutes to hours but it can be photochemically back-converted upon illumination with UVA light.