Light-Driven Chloride and Sulfate Pump with Two Different Transport Modes

Abstract

Ion pumps are membrane proteins that actively translocate ions by using energy. All known pumps bind ions in the resting state, and external energy allows ion transport through protein structural changes. The light-driven sodium-ion pump Krokinobacter eikastus rhodopsin 2 (KR2) is an exceptional case in which ion binding follows the energy input. In this study, we report another case of this unusual transport mode. The NTQ rhodopsin from Alteribacter aurantiacus (AaClR) is a natural light-driven chloride pump, in which the chloride ion binds to the resting state. AaClR is also able to pump sulfate ions, though the pump efficiency is much lower for sulfate ions than for chloride ions. Detailed spectroscopic analysis revealed no binding of the sulfate ion to the resting state of AaClR, indicating that binding of the substrate (sulfate ion) to the resting state is not necessary for active transport. This property of the AaClR sulfate pump is similar to that of the KR2 sodium pump. Photocycle dynamics of the AaClR sulfate pump resemble a non-functional cycle in the absence of anions. Despite this, flash photolysis and difference Fourier transform infrared spectroscopy suggest transient binding of the sulfate ion to AaClR. The molecular mechanism of this unusual active transport by AaClR is discussed.

Deep dive into the diversity and properties of rhodopsins in actinomycetes of the familyGeodermatophilaceae

“…To date, only three NTQ rhodopsins have been characterized in detail, namely, NM-R3 from Nonlabens marinus ( Flavobacteriia ) (Yoshizawa et al, 2014), FR from Fulvimarina pelagi ( Alphaproteobacteria) (Inoue et al, 2014) and AaClR from Alteribacter aurantiacus ( Bacillaceae ) (Singh et al, 2023). In addition, two identified but incompletely characterized rhodopsins from this group have been reported: PoClR from Parvularcula oceani ( Alphaproteobacteria ) (Inoue et al, 2016) and unnamed protein from Nonlabens sp.…”

“…DSM 44513 (GspTR) had absorption peak at 513 nm in 100 mM NaCl and was the most blue-shifted compared to the 24 NTQ rhodopsins available in the Karasuyama et al database (Karasuyama et al, 2018). The absorption peak of GspTR was the closest to that of FR rhodopsin from Fulvimarina pelagi (518 nm in 1660 mM NaCl) (Inoue et al, 2014) and of AaClR from Alteribacter aurantiacus (517 nm in 100 mM NaCl) (Singh et al, 2023), and showed significant distance from NM-R3 rhodopsin from Nonlabens marinus with absorption peak at 533.5 nm (Yoshizawa et al, 2014).…”

“…It is worth noting that most studies within the last two decades have focused on discovery and characterization of various microbial rhodopsins from marine and freshwater microorganisms (Chazan et al, 2022; Fuhrman et al, 2008; Chuon et al, 2021; Keffer et al, 2015; Nakamura et al, 2016; Sharma et al, 2008; Dwulit-Smith et al, 2018; Pushkarev and Béjà, 2016; Pushkarev et al, 2018; Hososhima et al, 2022; Singh et al, 2023). Little is known about distribution and the biological role of these proteins in microorganisms inhabiting terrestrial environments, especially extreme ones.…”

Photochemistry of the Retinal Chromophore in Microbial Rhodopsins

“…Microbial rhodopsins, also called Type 1 rhodopsins, are mostly present in unicellular microorganisms such as bacteria, archaea, algae, fungi, and protists. − Giant viruses have also been reported to possess microbial rhodopsins. − Animal rhodopsins, also called Type 2 rhodopsins, are present in only higher animals and work as light-dependent G protein-coupled receptor (GPCR) mediating visual and nonvisual signals or retinal photoisomerase, which catalyzes the isomerization reaction from all- trans -retinal (ATR) to the 11- cis form. ,, In contrast to animal rhodopsins, the functions of microbial rhodopsins are highly diversified. The most abundant types of microbial rhodopsins are light-driven ion pumps that actively transport, e.g., H + , , Na + , Cl – , and SO 4 2– . , In addition, while channelrhodopsins passively transport various ions as light-gated ion channels, , microbial rhodopsins with C-terminal enzyme or ion channel domains have been found; , further, there are many microbial rhodopsins whose functions remain unknown. − In 2018, the third class of rhodopsin, heliorhodopsin, was discovered through functional metagenomics . This class is phylogenetically distinct from both microbial and animal rhodopsins and has an inverted molecular orientation in which the N- and C-termini face the cytoplasmic and extracellular sides, respectively.…”