Parallel expression of alternate forms of psbA2 gene provides evidence for the existence of a targeted D1 repair mechanism in Synechocystis sp. PCC 6803

Cardiac Myosin Filaments are Maintained by Stochastic Protein Replacement

Myosin and myosin-binding protein C are exquisitely organized into giant filamentous macromolecular complexes within cardiac muscle sarcomeres, yet these proteins must be continually replaced to maintain contractile fidelity. The overall hypothesis that myosin filament structure is dynamic and allows for the stochastic replacement of individual components was tested in vivo, using a combination of mass spectrometry- and fluorescence-based proteomic techniques. Adult mice were fed a diet that marked all newly synthesized proteins with a stable isotope-labeled amino acid. The abundance of unlabeled and labeled proteins was quantified by high-resolution mass spectrometry over an 8-week period. The rates of change in the abundance of these proteins were well described by analytical models in which protein synthesis defined stoichiometry and protein degradation was governed by the stochastic selection of individual molecules. To test whether the whole myosin filaments or the individual components were selected for replacement, cardiac muscle was chemically skinned to remove the cellular membrane and myosin filaments were solubilized with ionic solutions. The composition of the filamentous and soluble fractions was quantified by mass spectrometry, and filament depolymerization was visualized by real-time fluorescence microscopy. Myosin molecules were preferentially extracted from ends of the filaments in the presence of the ionic solutions, and there was only a slight bias in the abundance of unlabeled molecules toward the innermost region on the myosin filaments. These data demonstrate for the first time that the newly synthesized myosin and myosin-binding protein C molecules are randomly mixed into preexisting thick filaments in vivo and the rate of mixing may not be equivalent along the length of the thick filament. These data collectively support a new model of cardiac myosin filament structure, with the filaments being dynamic macromolecular assemblies that allow for replacement of their components, rather than rigid bodies.

Photoactivation: The Light-Driven Assembly of the Water Oxidation Complex of Photosystem II

Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II. The assembly of the Mn4O5Ca requires light and involves a sequential process called photoactivation. This process harnesses the charge-separation of the photochemical reaction center and the coordination environment provided by the amino acid side chains of the protein to oxidize and organize the incoming manganese ions to form the oxo-bridged metal cluster capable of H2O-oxidation. Although most aspects of this assembly process remain poorly understood, recent advances in the elucidation of the crystal structure of the fully assembled cyanobacterial PSII complex help in the interpretation of the rich history of experiments designed to understand this process. Moreover, recent insights on the structure and stability of the constituent ions of the Mn4CaO5 cluster may guide future experiments. Here we consider the literature and suggest possible models of assembly including one involving single Mn(2+) oxidation site for all Mn but requiring ion relocation.

Accessibility controls selective degradation of photosystem II subunits by FtsH protease

The oxygen-evolving photosystem II (PSII) complex located in chloroplasts and cyanobacteria is sensitive to light-induced damage(1) that unless repaired causes reduction in photosynthetic capacity and growth. Although a potential target for crop improvement, the mechanism of PSII repair remains unclear. The D1 reaction center protein is the main target for photodamage(2), with repair involving the selective degradation of the damaged protein by FtsH protease(3). How a single damaged PSII subunit is recognized for replacement is unknown. Here, we have tested the dark stability of PSII subunits in strains of the cyanobacterium Synechocystis PCC 6803 blocked at specific stages of assembly. We have found that when D1, which is normally shielded by the CP43 subunit, becomes exposed in a photochemically active PSII complex lacking CP43, it is selectively degraded by FtsH even in the dark. Removal of the CP47 subunit, which increases accessibility of FtsH to the D2 subunit, induced dark degradation of D2 at a faster rate than that of D1. In contrast, CP47 and CP43 are resistant to degradation in the dark. Our results indicate that protease accessibility induced by PSII disassembly is an important determinant in the selection of the D1 and D2 subunits to be degraded by FtsH.

An atypical psbA gene encodes a sentinel D1 protein to form a physiologically relevant inactive photosystem II complex in cyanobacteria

Photosystem II, a large membrane-bound enzyme complex in cyanobacteria and chloroplasts, mediates light-induced oxidation of water to molecular oxygen. The D1 protein of PSII, encoded by the psbA gene, provides multiple ligands for cofactors crucial to this enzymatic reaction. Cyanobacteria contain multiple psbA genes that respond to various physiological cues and environmental factors. Certain unicellular cyanobacterial cells, such as Cyanothece sp. ATCC 51142, are capable of nitrogen fixation, a highly oxygen-sensitive process, by separating oxygen evolution from nitrogen fixation using a day-night cycle. We have shown that c-psbA4, one of the five psbA orthologs in this cyanobacterium, is exclusively expressed during nighttime. Remarkably, the corresponding D1 isoform has replacements of a number of amino acids that are essential ligands for the catalytic Mn4CaO5 metal center for water oxidation by PSII. At least 30 cyanobacterial strains, most of which are known to have nitrogen fixing abilities, have similar psbA orthologs. We expressed the c-psbA4 gene from Cyanothece 51142 in a 4E-3 mutant strain of the model non-nitrogen-fixing cyanobacterium Synechocystis sp. PCC 6803, which lacks any psbA gene. The resultant strain could not grow photoautotrophically. Moreover, these Synechocystis 6803 cells were incapable of PSII-mediated oxygen evolution. Based on our findings, we have named this physiologically relevant, unusual D1 isoform sentinel D1. Sentinel D1 represents a new class of D1 protein that, when incorporated in a PSII complex, ensures that PSII cannot mediate water oxidation, thus allowing oxygen-sensitive processes such as nitrogen fixation to occur in cyanobacterial cells.