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Carotenoid Pigment Is the Key to Photoprotection

Overexposure to sunlight, which is damaging to natural photosynthetic systems of green plants and cyanobacteria, is also expected to be damaging to artificial photosynthetic systems. Nature has solved the problem through a photoprotective mechanism called “nonphotochemical quenching,” in which excess solar energy is safely dissipated as heat. ALS researchers have recently discovered a surprising key event in this energy-quenching process.

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Translocation of the carotenoid pigment within a critical light-sensitive protein called the Orange Carotenoid Protein triggers a shifting of the protein from the light-absorbing orange state to the energy-quenching red state, providing cyanobacteria with protection from too much sunlight.

Photoprotection Through Shifting Pigments

Through photosynthesis, plants are able to harvest solar energy and convert it to chemical energy. Creating an efficient artificial version of photosynthesis would elevate solar power as the ultimate green and renewable source of electrical energy. However, if a sunlight-harvesting system becomes overloaded with absorbed solar energy, it most likely will suffer some form of damage.

The interactions of pigment molecules with one another and with proteins are of fundamental importance to the light harvesting and photoprotective functions essential to oxygenic photosynthesis. Researchers know that the distance between two pigment molecules is important for the transfer of energy, which means that knowing precisely a pigment molecule’s structural arrangement and its location at any given time in the process is critical.

At the ALS, researchers focused on the OCP, a pigment-binding protein in cyanobacteria that absorbs blue-green light and binds to the cyanobacterial light harvesting antenna to cause the dissipation of excess captured energy. Cyanobacteria are aquatic microbes that have been called the “architects of Earth’s atmosphere” because they generated most of the atmosphere’s oxygen during the Archaean and Proterozoic Eras. The chloroplasts that enable green plants to carry out photosynthesis today are descendants of ancient cyanobacteria.

The research team found that in cyanobacteria the energy-quenching mechanism is triggered by an unprecedented, large-scale (relatively speaking) movement from one location to another of the carotenoid pigment within a critical light-sensitive protein called the Orange Carotenoid Protein (OCP). As a result of this translocation, the carotenoid changes its shape slightly and interacts with a different set of amino acid neighbors, causing the protein to shift from an “orange” light-absorbing state to a “red” photoprotective state. This turns out to be an unanticipated molecular priming event in photoprotection.

Prior to this work, the assumption was that carotenoids were static, held in place by the protein scaffold. Having shown that the translocation of carotenoid within the protein is a functional trigger for photoprotection, scientists are now looking to other carotenoid-binding protein complexes to see if translocation could play a role in those as well. Understanding the dynamic function of carotenoids could be useful for the design of future artificial photosynthetic systems.

As orange-state OCP absorbs blue-green light, it undergoes structural changes that result in red-state OCP. Once the excess solar energy has been quenched, the OCP interacts with a second protein, the Fluorescence Recovery Protein (FRP), causing it to revert back to the light-harvesting, orange state. Using the protein crystallography capabilities of ALS Beamline 5.0.2, researchers were able to obtain crystal structures of these key photoprotective proteins.

The crystal structures revealed that OCP photoactivation is accompanied by a 12 Å translocation of the pigment within the protein and a dramatic reconfiguration of carotenoid-protein interactions. The researchers also identified the origin of the photochromic changes in the OCP triggered by light and revealed the structural determinants required for interaction with the light-harvesting antenna during photoprotection.

To confirm that this translocation actually occurs when OCP is in its natural solution environment and was not due to structural changes that the protein undergoes during crystal formation, researchers used ALS Beamline 5.3.1 and x-ray footprinting (XFP), a powerful technique in structural biology for the study of macromolecular structures and dynamics of proteins and nucleic acids in solution.

In any protein crystallography study, there is always the question of whether the crystallized protein represents the real-life protein, but with XFP you can look at proteins and/or nucleic acids in the solution state, and often under conditions that are close to physiological.

With the XFP study having confirmed the protein crystallography results as real, the next step for researchers will be to understand the detailed mechanism behind OCP’s interaction with the antenna to dissipate energy. Learning more about these might enable researchers to engineer smart photoprotection in cyanobacteria or even improve photosynthesis in green plants.

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Corie Ralston and Cheryl Kerfeld at ALS Beamline 5.0.2 where crystal structures of the Orange Carotenoid Protein in cyanobacteria were recorded as the protein transitioned from light-absorber to photoprotector. (Photo by Roy Kaltschmidt)

Contact: Cheryl Kerfield

Research conducted by: R. Leverenz (Michigan State University); M. Sutter (Michigan State University, Lawrence Berkeley National Laboratory); A. Wilson, A. Thurotte, D. Kirilovsky, and C. Bourcier de Carbon (Institut de Biologie et Technologies de Saclay); S. Gupta, C. Petzold, and  C. Ralston (Lawrence Berkeley National Laboratory); F. Perreau (Institut Jean-Pierre Bourgin); and C.A. Kerfeld (Michigan State University, Lawrence Berkeley National Laboratory, UC Berkeley).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.

Publication about this research: R. Leverenz, M. Sutter, A. Wilson, S. Gupta, A. Thurotte, C. Bourcier de Carbon, C. Petzold, C. Ralston, F. Perreau, D. Kirilovsky, and C.A. Kerfeld, “A 12 Å carotenoid translocation in a photoswitch associated with cyanobacterial photoprotection,” Science 348, 6242 (2015).

Adapted from the Berkeley Lab Science Short, “Orange is the New Red.”

ALS SCIENCE HIGHLIGHT #316

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