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

Ion pumps are membrane proteins that actively translocateionsby using energy. All known pumps bind ions in the resting state, andexternal 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 followsthe energy input. In this study, we report another case of this unusualtransport mode. The NTQ rhodopsin from Alteribacteraurantiacus (AaClR) is a natural light-driven chloridepump, in which the chloride ion binds to the resting state. AaClRis also able to pump sulfate ions, though the pump efficiency is muchlower for sulfate ions than for chloride ions. Detailed spectroscopicanalysis revealed no binding of the sulfate ion to the resting stateof AaClR, indicating that binding of the substrate (sulfate ion) tothe resting state is not necessary for active transport. This propertyof the AaClR sulfate pump is similar to that of the KR2 sodium pump.Photocycle dynamics of the AaClR sulfate pump resemble a non-functionalcycle in the absence of anions. Despite this, flash photolysis anddifference Fourier transform infrared spectroscopy suggest transientbinding of the sulfate ion to AaClR. The molecular mechanism of thisunusual active transport by AaClR is discussed.

Automated summary

AaClR is a natural light-driven chloride pump that binds chloride ions in the resting state. It is also able to pump sulfate ions, although with lower efficiency. The photocycle dynamics of the AaClR sulfate pump resemble a non-functional cycle, but there is evidence of transient binding of sulfate ions to AaClR. The molecular mechanism of this unusual active transport by AaClR is discussed.