The Psb32 protein aids in repairing photodamaged photosystem II in the cyanobacterium Synechocystis 6803

The phage shock protein (PSP) envelope stress response: discovery of novel partners and evolutionary history

Bacterial phage shock protein (PSP) systems stabilize the bacterial cell membrane and protect against envelope stress. These systems have been associated with virulence, but despite their critical roles, PSP components are not well characterized outside proteobacteria. Using comparative genomics and protein sequence-structure-function analyses, we systematically identified and analyzed PSP homologs, phyletic patterns, domain architectures, and gene neighborhoods. This approach underscored the evolutionary significance of the system, revealing that its core protein PspA (Snf7 in ESCRT outside bacteria) was present in the last universal common ancestor and that this ancestral functionality has since diversified into multiple novel, distinct PSP systems across life. Several novel partners of the PSP system were identified: (i) the Toastrack domain, likely facilitating assembly of sub-membrane stress-sensing and signaling complexes, (ii) the newly defined HTH-associated α-helical signaling domain-PadR-like transcriptional regulator pair system, and (iii) multiple independent associations with ATPase, CesT/Tir-like chaperone, and Band-7 domains in proteins thought to mediate sub-membrane dynamics. Our work also uncovered links between the PSP components and other domains, such as novel variants of SHOCT-like domains, suggesting roles in assembling membrane-associated complexes of proteins with disparate biochemical functions. Results are available at our interactive web app, https://jravilab.org/psp.IMPORTANCEPhage shock proteins (PSP) are virulence-associated, cell membrane stress-protective systems. They have mostly been characterized in Proteobacteria and Firmicutes. We now show that a minimal PSP system was present in the last universal common ancestor that evolved and diversified into newly identified functional contexts. Recognizing the conservation and evolution of PSP systems across bacterial phyla contributes to our understanding of stress response mechanisms in prokaryotes. Moreover, the newly discovered PSP modularity will likely prompt new studies of lineage-specific cell envelope structures, lifestyles, and adaptation mechanisms. Finally, our results validate the use of domain architecture and genetic context for discovery in comparative genomics.

1H, 15N and 13C backbone and side chain solution NMR assignments of the TPM domain-containing protein of the thermophilic bacterium Rhodothermus marinus

The InterPro family IPR007621 TPM_phosphatase is a widely conserved family of protein domains found in prokaryotes, plants and invertebrates. Despite similar predicted protein folding, members of this family are involved in different cellular processes. In recent years, the structural and biochemical characterization of evolutionarily divergent TPM domains has shown their ability to hydrolyze phosphate groups of different substrates. However, there are still inaccurate functional annotations and uncertain relationships between the structure and function of this domain family. We here report the 1H, 13C, and 15N backbone and sidechain resonances of the TPM domain of a predicted TPM domain-containing protein of the thermophilic bacterium Rhodothermus marinus. These data will lay the groundwork for future NMR-based investigations, contributing to a thorough comprehension of the intricate aspects governing the interplay between structure and function of TPM domains. Additionally, they will unlock opportunities to explore dynamic structural changes, providing valuable insights into the molecular mechanisms underlying the evolutionary adaptations to extreme environmental conditions within this protein family.

Sll1252 Coordinates Electron Transport between Plastoquinone and Cytochrome b6/f Complex in Synechocystis PCC 6803

A mutant, Δsll1252ins, was generated to functionally characterize Sll1252. Δsll1252ins exhibited a slow-growth phenotype at 70 µmol photons m-2 s-1 and glucose sensitivity. In Δsll1252ins, the rate of PSII activity was not affected, whereas the whole chain electron transport activity was reduced by 45%. The inactivation of sll1252 led to the upregulation of genes, which were earlier reported to be induced in DBMIB-treated wild-type, suggesting that Sll1252 may be involved in electron transfer from the reduced-PQ pool to Cyt b6/f. The inhibitory effect of DCMU on PSII activity was similar in both wild-type and Δsll1252ins. However, the concentration of DBMIB for 50% inhibition of whole chain electron transport activity was 140 nM for Δsll1252ins and 300 nM for wild-type, confirming the site of action of Sll1252. Moreover, the elevated level of the reduced-PQ pool in Δsll1252ins supports that Sll1252 functions between the PQ pool and Cyt b6/f. Interestingly, we noticed that Δsll1252ins reverted to wild-type phenotype by insertion of natural transposon, ISY523, at the disruption site. Δsll1252-Ntrn, expressing only the C-terminal region of Sll1252, exhibited a slow-growth phenotype and disorganized thylakoid structure compared to wild-type and Δsll1252-Ctrn (expressing only the N-terminal region). Collectively, our data suggest that Sll1252 regulates electron transfer between the PQ pool and the Cyt b6/f complex in the linear photosynthetic electron transport chain via coordinated function of both the N- and C-terminal regions of Sll1252.

Accumulation of Cyanobacterial Photosystem II Containing the 'Rogue' D1 Subunit Is Controlled by FtsH Protease and Synthesis of the Standard D1 Protein

Unicellular diazotrophic cyanobacteria contribute significantly to the photosynthetic productivity of the ocean and the fixation of molecular nitrogen, with photosynthesis occurring during the day and nitrogen fixation during the night. In species like Crocosphaera watsonii WH8501, the decline in photosynthetic activity in the night is accompanied by the disassembly of oxygen-evolving photosystem II (PSII) complexes. Moreover, in the second half of the night phase, a small amount of rogue D1 (rD1), which is related to the standard form of the D1 subunit found in oxygen-evolving PSII, but of unknown function, accumulates but is quickly degraded at the start of the light phase. We show here that the removal of rD1 is independent of the rD1 transcript level, thylakoid redox state and trans-thylakoid pH but requires light and active protein synthesis. We also found that the maximal level of rD1 positively correlates with the maximal level of chlorophyll (Chl) biosynthesis precursors and enzymes, which suggests a possible role for rogue PSII (rPSII) in the activation of Chl biosynthesis just before or upon the onset of light, when new photosystems are synthesized. By studying strains of Synechocystis PCC 6803 expressing Crocosphaera rD1, we found that the accumulation of rD1 is controlled by the light-dependent synthesis of the standard D1 protein, which triggers the fast FtsH2-dependent degradation of rD1. Affinity purification of FLAG-tagged rD1 unequivocally demonstrated the incorporation of rD1 into a non-oxygen-evolving PSII complex, which we term rPSII. The complex lacks the extrinsic proteins stabilizing the oxygen-evolving Mn4CaO5 cluster but contains the Psb27 and Psb28-1 assembly factors.

Asymmetric survival in single-cell lineages of cyanobacteria in response to photodamage

Oxygenic photosynthesis is driven by the coupled action of the light-dependent pigment-protein complexes, photosystem I and II, located within the internal thylakoid membrane system. However, photosystem II is known to be prone to photooxidative damage. Thus, photosynthetic organisms have evolved a repair cycle to continuously replace the damaged proteins in photosystem II. However, it has remained difficult to deconvolute the damage and repair processes using traditional ensemble approaches. Here, we demonstrate an automated approach using time-lapse fluorescence microscopy and computational image analysis to study the dynamics and effects of photodamage in single cells at subcellular resolution in cyanobacteria. By growing cells in a two-dimensional layer, we avoid shading effects, thereby generating uniform and reproducible growth conditions. Using this platform, we analyzed the growth and physiology of multiple strains simultaneously under defined photoinhibitory conditions stimulated by UV-A light. Our results reveal an asymmetric cellular response to photodamage between sibling cells and the generation of an elusive subcellular structure, here named a 'photoendosome,' derived from the thylakoid which could indicate the presence of a previously unknown photoprotective mechanism. We anticipate these results to be a starting point for further studies to better understand photodamage and repair at the single-cell level.

Proximity Labeling Facilitates Defining the Proteome Neighborhood of Photosystem II Oxygen Evolution Complex in a Model Cyanobacterium

Ascorbate peroxidase (APEX)-based proximity labeling coupled with mass spectrometry has a great potential for spatiotemporal identification of proteins proximal to a protein complex of interest. Using this approach is feasible to define the proteome neighborhood of important protein complexes in a popular photosynthetic model cyanobacterium Synechocystis sp. PCC6803 (hereafter named as Synechocystis). To this end, we developed a robust workflow for APEX2-based proximity labeling in Synechocystis and used the workflow to identify proteins proximal to the photosystem II (PS II) oxygen evolution complex (OEC) through fusion APEX2 with a luminal OEC subunit, PsbO. In total, 38 integral membrane proteins (IMPs) and 93 luminal proteins were identified as proximal to the OEC. A significant portion of these proteins are involved in PS II assembly, maturation, and repair, while the majority of the rest were not previously implicated with PS II. The IMPs include subunits of PS II and cytochrome b6/f, but not of photosystem I (except for PsaL) and ATP synthases, suggesting that the latter two complexes are spatially separated from the OEC with a distance longer than the APEX2 labeling radius. Besides, the topologies of six IMPs were successfully predicted because their lumen-facing regions exclusively contain potential APEX2 labeling sites. The luminal proteins include 66 proteins with a predicted signal peptide and 57 proteins localized also in periplasm, providing important targets to study the regulation and selectivity of protein translocation. Together, we not only developed a robust workflow for the application of APEX2-based proximity labeling in Synechocystis and showcased the feasibility to define the neighborhood proteome of an important protein complex with a short radius but also discovered a set of the proteins that potentially interact with and regulate PS II structure and function.

Advances in the Understanding of the Lifecycle of Photosystem II

Photosystem II is a light-driven water-plastoquinone oxidoreductase present in cyanobacteria, algae and plants. It produces molecular oxygen and protons to drive ATP synthesis, fueling life on Earth. As a multi-subunit membrane-protein-pigment complex, Photosystem II undergoes a dynamic cycle of synthesis, damage, and repair known as the Photosystem II lifecycle, to maintain a high level of photosynthetic activity at the cellular level. Cyanobacteria, oxygenic photosynthetic bacteria, are frequently used as model organisms to study oxygenic photosynthetic processes due to their ease of growth and genetic manipulation. The cyanobacterial PSII structure and function have been well-characterized, but its lifecycle is under active investigation. In this review, advances in studying the lifecycle of Photosystem II in cyanobacteria will be discussed, with a particular emphasis on new structural findings enabled by cryo-electron microscopy. These structural findings complement a rich and growing body of biochemical and molecular biology research into Photosystem II assembly and repair.

Proximity-based proteomics reveals the thylakoid lumen proteome in the cyanobacterium Synechococcus sp. PCC 7002

Cyanobacteria possess unique intracellular organization. Many proteomic studies have examined different features of cyanobacteria to learn about the intracellular structures and their respective functions. While these studies have made great progress in understanding cyanobacterial physiology, the conventional fractionation methods used to purify cellular structures have limitations; specifically, certain regions of cells cannot be purified with existing fractionation methods. Proximity-based proteomics techniques were developed to overcome the limitations of biochemical fractionation for proteomics. Proximity-based proteomics relies on spatiotemporal protein labeling followed by mass spectrometry of the labeled proteins to determine the proteome of the region of interest. We performed proximity-based proteomics in the cyanobacterium Synechococcus sp. PCC 7002 with the APEX2 enzyme, an engineered ascorbate peroxidase. We determined the proteome of the thylakoid lumen, a region of the cell that has remained challenging to study with existing methods, using a translational fusion between APEX2 and PsbU, a lumenal subunit of photosystem II. Our results demonstrate the power of APEX2 as a tool to study the cell biology of intracellular features and processes, including photosystem II assembly in cyanobacteria, with enhanced spatiotemporal resolution.

Comparison of photosynthetic performances of marine picocyanobacteria with different configurations of the oxygen-evolving complex

The extrinsic PsbU and PsbV proteins are known to play a critical role in stabilizing the Mn₄CaO₅ cluster of the PSII oxygen-evolving complex (OEC). However, most isolates of the marine cyanobacterium Prochlorococcus naturally miss these proteins, even though they have kept the main OEC protein, PsbO. A structural homology model of the PSII of such a natural deletion mutant strain (P. marinus MED4) did not reveal any obvious compensation mechanism for this lack. To assess the physiological consequences of this unusual OEC, we compared oxygen evolution between Prochlorococcus strains missing psbU and psbV (PCC 9511 and SS120) and two marine strains possessing these genes (Prochlorococcus sp. MIT9313 and Synechococcus sp. WH7803). While the low light-adapted strain SS120 exhibited the lowest maximal O₂ evolution rates (P_max per divinyl-chlorophyll a, per cell or per photosystem II) of all four strains, the high light-adapted strain PCC 9511 displayed even higher P^Chl_max and P^PSII_max at high irradiance than Synechococcus sp. WH7803. Furthermore, thermoluminescence glow curves did not show any alteration in the B-band shape or peak position that could be related to the lack of these extrinsic proteins. This suggests an efficient functional adaptation of the OEC in these natural deletion mutants, in which PsbO alone is seemingly sufficient to ensure proper oxygen evolution. Our study also showed that Prochlorococcus strains exhibit negative net O₂ evolution rates at the low irradiances encountered in minimum oxygen zones, possibly explaining the very low O₂ concentrations measured in these environments, where Prochlorococcus is the dominant oxyphototroph.

Structural and functional characterization of a cold-adapted stand-alone TPM domain reveals a relationship between dynamics and phosphatase activity

The TPM domain constitutes a family of recently characterized protein domains that are present in most living organisms. Although some progress has been made in understanding the cellular role of TPM-containing proteins, the relationship between structure and function is not clear yet. We have recently solved the solution and crystal structure of one TPM domain (BA42) from the Antarctic bacterium Bizionia argentinensis. In this work, we demonstrate that BA42 has phosphoric-monoester hydrolase activity. The activity of BA42 is strictly dependent on the binding of divalent metals and retains nearly 70% of the maximum at 4 °C, a typical characteristic of cold-adapted enzymes. From HSQC, ¹⁵ N relaxation measurements, and molecular dynamics studies, we determine that the flexibility of the crossing loops was associated to the protein activity. Thermal unfolding experiments showed that the local increment in flexibility of Mg²⁺ -bound BA42, when compared with Ca²⁺ -bound BA42, is associated to a decrease in global protein stability. Finally, through mutagenesis experiments, we unambiguously demonstrate that the region comprising the metal-binding site participates in the catalytic mechanism. The results shown here contribute to the understanding of the relationship between structure and function of this new family of TPM domains providing important cues on the regulatory role of Mg²⁺ and Ca²⁺ and the molecular mechanism underlying enzyme activity at low temperatures.

Analysis of photosystem II biogenesis in cyanobacteria

Photosystem II (PSII), a large multisubunit membrane protein complex found in the thylakoid membranes of cyanobacteria, algae and plants, catalyzes light-driven oxygen evolution from water and reduction of plastoquinone. Biogenesis of PSII requires coordinated assembly of at least 20 protein subunits, as well as incorporation of various organic and inorganic cofactors. The stepwise assembly process is facilitated by numerous protein factors that have been identified in recent years. Further analysis of this process requires the development or refinement of specific methods for the identification of novel assembly factors and, in particular, elucidation of the unique role of each. Here we summarize current knowledge of PSII biogenesis in cyanobacteria, focusing primarily on the impact of methodological advances and innovations. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Conrad Mullineaux.

Cloning, expression, crystallization and preliminary X-ray studies of a superfolder GFP fusion of cyanobacterial Psb32

A fusion of Psb32 from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 (TePsb32) with superfolder GFP was created for enhanced solubility and improved detection and purification. The fusion protein readily formed large hexagonal crystals belonging to space group P6₁22. A full data set extending to 2.3 Å resolution was collected at the Swiss Light Source. The phase problem could be solved by using only the sfGFP fusion partner or by using GFP and AtTLP18.3 from Arabidopsis thaliana as search models. Based on this expression construct, a versatile library of 24 vectors combining four different superfolder GFP variants and three affinity tags was generated to facilitate expression and screening of fluorescent fusion proteins.

Identification of low-molecular-weight compounds inhibiting growth of corynebacteria: potential lead compounds for antibiotics

The bacterial genus Corynebacteria contains several pathogenic species that cause diseases such as diphtheria in humans and "cheesy gland" in goats and sheep. Thus, identifying new therapeutic targets to treat Corynebacteria infections is both medically and economically important. CG2496, a functionally uncharacterized protein from Corynebacterium glutamicum, was evaluated using an NMR ligand-affinity screen. A total of 11 compounds from a library of 460 biologically active compounds were shown to selectively bind CG2496 in a highly conserved region of the protein. The best binder was identified to be methiothepin (KD =54 ± 19 µM), an FDA-approved serotonin receptor antagonist. Methiothepin was also shown to inhibit the growth of C. glutamicum, but not bacteria that lack CG2496 homologs. Our results suggest that CG2496 is a novel therapeutic target and methiothepin is a potential lead compound or structural scaffold for developing new antibiotics specifically targeting Corynebacteria.

Structural, functional and auxiliary proteins of photosystem II

Photosystem II (PSII) is the water-splitting enzyme complex of photosynthesis and consists of a large number of protein subunits. Most of these proteins have been structurally and functionally characterized, although there are differences between PSII of plants, algae and cyanobacteria. Here we catalogue all known PSII proteins giving a brief description, where possible of their genetic origin, physical properties, structural relationships and functions. We have also included details of auxiliary proteins known at present to be involved in the in vivo assembly, maintenance and turnover of PSII and which transiently bind to the reaction centre core complex. Finally, we briefly give details of the proteins which form the outer light-harvesting systems of PSII in different types of organisms.

Photosystem II assembly: from cyanobacteria to plants

Photosystem II (PSII) is an integral-membrane, multisubunit complex that initiates electron flow in oxygenic photosynthesis. The biogenesis of this complex machine involves the concerted assembly of at least 20 different polypeptides as well as the incorporation of a variety of inorganic and organic cofactors. Many factors have recently been identified that constitute an integrative network mediating the stepwise assembly of PSII components. One recurring theme is the subcellular organization of the assembly process in specialized membranes that form distinct biogenesis centers. Here, we review our current knowledge of the molecular components and events involved in PSII assembly and their high degree of evolutionary conservation.

Evidence for nucleotide-dependent processes in the thylakoid lumen of plant chloroplasts--an update

The thylakoid lumen is an aqueous chloroplast compartment enclosed by the thylakoid membrane network. Bioinformatic and proteomic studies indicated the existence of 80-90 thylakoid lumenal proteins in Arabidopsis thaliana, having photosynthetic, non-photosynthetic or unclassified functions. None of the identified lumenal proteins had canonical nucleotide-binding motifs. It was therefore suggested that, in contrast to the chloroplast stroma harboring nucleotide-dependent enzymes and other proteins, the thylakoid lumen is a nucleotide-free compartment. Based on recent findings, we provide here an updated view about the presence of nucleotides in the thylakoid lumen of plant chloroplasts, and their role in function and dynamics of photosynthetic complexes.

Photosystem II, a growing complex: updates on newly discovered components and low molecular mass proteins

Photosystem II is a unique complex capable of absorbing light and splitting water. The complex has been thoroughly studied and to date there are more than 40 proteins identified, which bind to the complex either stably or transiently. Another special feature of this complex is the unusually high content of low molecular mass proteins that represent more than half of the proteins. In this review we summarize the recent findings on the low molecular mass proteins (<15kDa) and present an overview of the newly identified components as well. We have also performed co-expression analysis of the genes encoding PSII proteins to see if the low molecular mass proteins form a specific sub-group within the Photosystem II complex. Interestingly we found that the chloroplast-localized genes encoding PSII proteins display a different response to environmental and stress conditions compared to the nuclear localized genes. This article is part of a Special Issue entitled: Photosystem II.