Abstract
Rhodopsins are light-activated chromoproteins that mediate signaling processes via transducer proteins or promote active or passive ion transport as ion pumps or directly light-activated channels. Here, we provide spectroscopic characterization of a rhodopsin from the Chlamydomonas eyespot. It belongs to a recently discovered but so far uncharacterized family of histidine kinase rhodopsins (HKRs). These are modular proteins consisting of rhodopsin, a histidine kinase, a response regulator, and in some cases an effector domain such as an adenylyl or guanylyl cyclase, all encoded in a single protein as a two-component system. The recombinant rhodopsin fragment, Rh, of HKR1 is a UVA receptor (λ(max) = 380 nm) that is photoconverted by UV light into a stable blue light-absorbing meta state Rh-Bl (λ(max) = 490 nm). Rh-Bl is converted back to Rh-UV by blue light. Raman spectroscopy revealed that the Rh-UV chromophore is in an unusual 13-cis,15-anti configuration, which explains why the chromophore is deprotonated. The excited state lifetime of Rh-UV is exceptionally stable, probably caused by a relatively unpolar retinal binding pocket, converting into the photoproduct within about 100 ps, whereas the blue form reacts 100 times faster. We propose that the photochromic HKR1 plays a role in the adaptation of behavioral responses in the presence of UVA light.
Highlights
Microbial rhodopsins in Chlamydomonas are employed for photoorientation and developmental processes
Localization of HKR1 in the Eyespot of Chlamydomonas—As a first action we produced antisera against the rhodopsin fragment (Rh) and the response regulator portion (RR) of HKR1 to localize the protein in the alga
The localization of HKR1 within the eyespot of C. reinhardtii originally surprised us because phototaxis was thought to be mediated exclusively by the channelrhodopsins ChR1 and ChR2 [26, 27], and control of photobehavior has been considered the major if not exclusive role of the eye
Summary
Microbial rhodopsins in Chlamydomonas are employed for photoorientation and developmental processes. We provide spectroscopic characterization of a rhodopsin from the Chlamydomonas eyespot It belongs to a recently discovered but so far uncharacterized family of histidine kinase rhodopsins (HKRs). Rhodopsins with absorption at the red edge of the spectrum (XL-iodopsins) are found in flies, mollusks, fish, and in some birds At this spectral range, thermal isomerization increases, producing significant background at high rhodopsin concentrations [3]. A previously undescribed class of rhodopsin sequences has been found in several algal genomes including C. reinhardtii [8] These rhodopsins are directly connected via the C terminus to a histidine kinase and a response regulator, defining a novel rhodopsin subfamily of histidine-kinase rhodopsins (HKRs).. It has been suggested that these HKRs are involved in cell cycle regulation
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