Rhodopsin-guanylyl cyclases (RhGC) are unique photoreceptors recently discovered in Blastocladiomycota fungi . In RhGCs the light-sensitive microbial rhodopsin domain is covalently linked to a type III guanylyl cyclase. Guanylyl cyclases form the second messenger cGMP, which together with cAMP regulates a variety of biological processes [2–12]. Due to their architecture, RhGCs are classified as microbial enzyme rhodopsins. In the present work, the five newly discovered RhGCs were characterized using different biochemical and biophysical methods. Electrophysiological measurements provided indirect evidence for green light-activated cGMP synthesis of the RhGCs from Blastocladiella emersonii (Be) and Catenaria anguillulae (Ca). The light-activated guanylyl cyclase function could be confirmed by ELISA experiments and after purification of these photoreceptors. Green illumination led to a 100-fold or 200-fold increase in cGMP with a vmax of 1.8 or 11.6 µmol/min/mg(protein) for BeRhGC or CaRhGC. In the dark the cGMP concentration remained at the level of control cells for both photoreceptors. A kinetic analysis of the isolated guanylyl cyclase domains (Be/CaGC) revealed the constitutive activity of the enzymatic domain, which was 3-6x reduced compared to the full-length photoreceptors. A spectroscopic characterization of the Be/Ca rhodopsin domains allowed the identification of photocycle intermediates, which are typical for microbial rhodopsins. The M-intermediates decayed slowly with a τ ~ 100 ms for BeRh and τ ~ 500 ms for CaRh. The Be/Ca rhodopsin domains were mutated to change the kinetic and spectroscopic parameters of the photoreceptors. In addition, the substrate specificity of the RhGCs was switched to ATP by a double mutation (E497K/C566D) in the catalytic domain. The light-induced cAMP synthesis of the generated rhodopsin-adenylyl cyclases (Be/CaRhACs) was shown in Xenopus oocytes and after purification of the proteins. Compared to BeRhAC, CaRhAC showed an increased light-to-dark activity (6x) and a decreased activity in darkness (5.5x). To get further insight into the recently discovered RhGCs, the isolated cyclase domains, Be/CaGC and CaAC, were crystallized in the presence of NTP analogues. High-resolution monomeric GC structures without a bound ligand were produced. Additionally, a 2.25 Å structure of the mutated cyclase, CaAC, with the ATP analogue ATPαS was solved. The CaAC structure shows an antiparallel arrangement of the dimer subunits and the nucleotide base is bound by the previously mutated residues. Due to the similarity to other type III cyclases, a classical reaction sequence for RhGCs can be deduced. Finally, the applicability of Ca/BeRhGC and CaRhAC was tested in hippocampal rat neurons and CHO cells. These application-oriented approaches show that both RhGCs and YFP-CaRhAC can be used as optogenetic tools to precisely control cGMP and cAMP with light.
|Advisor:||Peter , Hegemann , Birte , Höcker , Holger , Dobbek|
|School:||Humboldt Universitaet zu Berlin (Germany)|
|Source:||DAI-C 81/7(E), Dissertation Abstracts International|
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