The Plant Cell, Vol. 28: 826, April 2016, www.plantcell.org ã 2016 American Society of Plant Biologists. All rights reserved.
IN BRIEF
Special Delivery: A Crucial Protein That Transports Manganese to the Oxygen-Evolving Complex of Photosystem II OPEN
Deep in the thylakoid membrane, the key process that produces breathable oxygen in the Earth’s atmosphere takes place, namely, the splitting of water molecules and the subsequent release of molecular oxygen. This process, which occurs in the oxygen-evolving complex (OEC) of the intricately arranged multiprotein and pigment complex known as photosystem II (PSII), is driven by the strong electrochemical potential generated from the capture and conversion of visible light to chemical energy. First, chlorophyll molecules in PSII absorb photons and lose electrons. These electrons are then passed through an electron transport chain, creating redox potential strong enough to oxidize water, which leads to the evolution of molecular oxygen and the release of protons into the thylakoid lumen. This proton gradient is a major contributor to the proton motive force (PMF), which is used to biosynthesize ATP from ADP via ATP synthase. The OEC contains an inorganic, flexible Mn4CaO5 cluster resembling a distorted chair, which is bound to a pocket formed by six amino acids from the D1 core subunit protein and one amino acid from the lightharvesting protein CP43 (reviewed in Shen, 2015). The Mn4CaO5 cluster is stabilized, at least in part, by the PsbO subunits of the OEC. The psbo mutants have pale-green leaves and a reduced growth rate, highlighting the importance of these proteins (Murakami et al., 2002). Maintenance of the highly dynamic Mn4CaO5 cluster also requires the delivery of a constant supply of the proper levels of Mn21 and Ca21. However, due to technical constraints, little is known about how Mn21 and Ca21 (and other ions) are transported to their destinations within the cell. Through rigorous analysis of an Arabidopsis thaliana mutant impaired in photosynthesis, Schneider et al. (2016) identified PAM71 (PHOTOSYNTHESIS AFFECTED MUTANT71), a protein involved in Mn21 transOPEN
Articles can be viewed without a subscription.
www.plantcell.org/cgi/doi/10.1105/tpc.16.00269
The PAM mutant exhibits altered photosynthetic parameters. Wild-type (WT), mutant (pam71-1 and pam71-2), and complemented (pam71-1Pro35S: PAM71-GFP) plants showing minimal chlorophyll a fluorescence (F0) and maximum quantum yield of PSII (FV/FM). Color scale indicates signal intensities. (Reprinted from Schneider et al. [2016], Figure 1B.)
port into the thylakoid lumen. Like psbo mutants, pam71 has pale-green leaves and impaired growth. In addition, pam71 exhibits defects in PSII, such as increased minimal chlorophyll a fluorescence (F0) and reduced maximum quantum yield of PSII (F V /F M ) (see figure), as well as reduced amounts of most PSII subunits and starch granules. The oxygen evolution rate in pam71 is only 20% of normal levels. PAM71 is not involved in OEC assembly, as it failed to interact with OEC subunits, such as PsbO, in a coimmunoprecipitation experiment. Mass spectrometry revealed a marked reduction in Mn21 binding to various PSII complexes in the mutant, especially the Mn4CaO5 cluster, although total Mn21 levels were normal in psbo mutant leaves. PAM71 is highly conserved in photosynthetic organisms and is homologous to yeast and human proteins involved in maintaining Ca21 homeostasis. Microscopy and biochemical analysis revealed that PAM71 is integrated in the thylakoid membrane. The flow of ions across the thylakoid membrane is thought to counterbalance
proton pumping into the lumen, thereby altering the PMF. Interestingly, the pam71 mutant has an elevated PMF across the thylakoid membrane, suggesting the presence of an ion imbalance, with cations accumulating in the lumen. Indeed, radiolabeled 45Ca21 accumulated in the thylakoid lumen of illuminated pam71 cells, whereas 54Mn21 tended to accumulate in the stroma. Finally, the diminished functionality of PSII in the mutant was restored by exogenous treatment with Mn21 (but not Ca21), which was not the case for a psbo double mutant. Mn21 supplementation also restored the impaired photosynthesis of a homologous Chlamydomonas reinhardtii mutant. These findings point to a fundamental role for PAM71 in Mn21 transport across the thylakoid membrane to sustain the formation of the Mn4CaO5 cluster (perhaps acting as a Mn21/proton exchanger), a vital process for both plants and our atmosphere.
Jennifer Lockhart Science Editor [email protected] ORCID ID: 0000-0002-1394-8947
REFERENCES Murakami, R., Ifuku, K., Takabayashi, A., Shikanai, T., Endo, T., and Sato, F. (2002). Characterization of an Arabidopsis thaliana mutant with impaired psbO, one of two genes encoding extrinsic 33-kDa proteins in photosystem II. FEBS Lett. 523: 138–142. Schneider, A., et al. (2016). The evolutionarily conserved protein PHOTOSYNTHESIS AFFECTED MUTANT71 is required for efficient manganese uptake at the thylakoid membrane in Arabidopsis. Plant Cell 28: 892–910. Shen, J.R. (2015). The structure of photosystem II and the mechanism of water oxidation in photosynthesis. Annu. Rev. Plant Biol. 66: 23–48.
IN BRIEF
Special Delivery: A Crucial Protein That Transports Manganese to the Oxygen-Evolving Complex of Photosystem II OPEN
Deep in the thylakoid membrane, the key process that produces breathable oxygen in the Earth’s atmosphere takes place, namely, the splitting of water molecules and the subsequent release of molecular oxygen. This process, which occurs in the oxygen-evolving complex (OEC) of the intricately arranged multiprotein and pigment complex known as photosystem II (PSII), is driven by the strong electrochemical potential generated from the capture and conversion of visible light to chemical energy. First, chlorophyll molecules in PSII absorb photons and lose electrons. These electrons are then passed through an electron transport chain, creating redox potential strong enough to oxidize water, which leads to the evolution of molecular oxygen and the release of protons into the thylakoid lumen. This proton gradient is a major contributor to the proton motive force (PMF), which is used to biosynthesize ATP from ADP via ATP synthase. The OEC contains an inorganic, flexible Mn4CaO5 cluster resembling a distorted chair, which is bound to a pocket formed by six amino acids from the D1 core subunit protein and one amino acid from the lightharvesting protein CP43 (reviewed in Shen, 2015). The Mn4CaO5 cluster is stabilized, at least in part, by the PsbO subunits of the OEC. The psbo mutants have pale-green leaves and a reduced growth rate, highlighting the importance of these proteins (Murakami et al., 2002). Maintenance of the highly dynamic Mn4CaO5 cluster also requires the delivery of a constant supply of the proper levels of Mn21 and Ca21. However, due to technical constraints, little is known about how Mn21 and Ca21 (and other ions) are transported to their destinations within the cell. Through rigorous analysis of an Arabidopsis thaliana mutant impaired in photosynthesis, Schneider et al. (2016) identified PAM71 (PHOTOSYNTHESIS AFFECTED MUTANT71), a protein involved in Mn21 transOPEN
Articles can be viewed without a subscription.
www.plantcell.org/cgi/doi/10.1105/tpc.16.00269
The PAM mutant exhibits altered photosynthetic parameters. Wild-type (WT), mutant (pam71-1 and pam71-2), and complemented (pam71-1Pro35S: PAM71-GFP) plants showing minimal chlorophyll a fluorescence (F0) and maximum quantum yield of PSII (FV/FM). Color scale indicates signal intensities. (Reprinted from Schneider et al. [2016], Figure 1B.)
port into the thylakoid lumen. Like psbo mutants, pam71 has pale-green leaves and impaired growth. In addition, pam71 exhibits defects in PSII, such as increased minimal chlorophyll a fluorescence (F0) and reduced maximum quantum yield of PSII (F V /F M ) (see figure), as well as reduced amounts of most PSII subunits and starch granules. The oxygen evolution rate in pam71 is only 20% of normal levels. PAM71 is not involved in OEC assembly, as it failed to interact with OEC subunits, such as PsbO, in a coimmunoprecipitation experiment. Mass spectrometry revealed a marked reduction in Mn21 binding to various PSII complexes in the mutant, especially the Mn4CaO5 cluster, although total Mn21 levels were normal in psbo mutant leaves. PAM71 is highly conserved in photosynthetic organisms and is homologous to yeast and human proteins involved in maintaining Ca21 homeostasis. Microscopy and biochemical analysis revealed that PAM71 is integrated in the thylakoid membrane. The flow of ions across the thylakoid membrane is thought to counterbalance
proton pumping into the lumen, thereby altering the PMF. Interestingly, the pam71 mutant has an elevated PMF across the thylakoid membrane, suggesting the presence of an ion imbalance, with cations accumulating in the lumen. Indeed, radiolabeled 45Ca21 accumulated in the thylakoid lumen of illuminated pam71 cells, whereas 54Mn21 tended to accumulate in the stroma. Finally, the diminished functionality of PSII in the mutant was restored by exogenous treatment with Mn21 (but not Ca21), which was not the case for a psbo double mutant. Mn21 supplementation also restored the impaired photosynthesis of a homologous Chlamydomonas reinhardtii mutant. These findings point to a fundamental role for PAM71 in Mn21 transport across the thylakoid membrane to sustain the formation of the Mn4CaO5 cluster (perhaps acting as a Mn21/proton exchanger), a vital process for both plants and our atmosphere.
Jennifer Lockhart Science Editor [email protected] ORCID ID: 0000-0002-1394-8947
REFERENCES Murakami, R., Ifuku, K., Takabayashi, A., Shikanai, T., Endo, T., and Sato, F. (2002). Characterization of an Arabidopsis thaliana mutant with impaired psbO, one of two genes encoding extrinsic 33-kDa proteins in photosystem II. FEBS Lett. 523: 138–142. Schneider, A., et al. (2016). The evolutionarily conserved protein PHOTOSYNTHESIS AFFECTED MUTANT71 is required for efficient manganese uptake at the thylakoid membrane in Arabidopsis. Plant Cell 28: 892–910. Shen, J.R. (2015). The structure of photosystem II and the mechanism of water oxidation in photosynthesis. Annu. Rev. Plant Biol. 66: 23–48.
