Compartmentalization of the protein repair machinery in photosynthetic membranes

Abstract

A crucial component of protein homeostasis in cells is the repair of damaged proteins. The repair of oxygen-evolving photosystem II (PS II) supercomplexes in plant chloroplasts is a prime example of a very efficient repair process that evolved in response to the high vulnerability of PS II to photooxidative damage, exacerbated by high-light (HL) stress. Significant progress in recent years has unraveled individual components and steps that constitute the PS II repair machinery, which is embedded in the thylakoid membrane system inside chloroplasts. However, an open question is how a certain order of these repair steps is established and how unwanted back-reactions that jeopardize the repair efficiency are avoided. Here, we report that spatial separation of key enzymes involved in PS II repair is realized by subcompartmentalization of the thylakoid membrane, accomplished by the formation of stacked grana membranes. The spatial segregation of kinases, phosphatases, proteases, and ribosomes ensures a certain order of events with minimal mutual interference. The margins of the grana turn out to be the site of protein degradation, well separated from active PS II in grana core and de novo protein synthesis in unstacked stroma lamellae. Furthermore, HL induces a partial conversion of stacked grana core to grana margin, which leads to a controlled access of proteases to PS II. Our study suggests that the origin of grana in evolution ensures high repair efficiency, which is essential for PS II homeostasis.photosynthesis | photoinhibition | PS II repair cycle | thylakoid membrane | grana margin R epair of damaged protein complexes is essential for the survival of all living organisms. One of nature's most efficient repair machineries is localized in photosynthetic thylakoid membranes of plants, which can turn over the total pool of the watersplitting photosystem II (PS II) supercomplex in less than 1 h (1). This remarkable potential for protein repair is crucial for the survival and fitness of plants because photodamage by reactive oxygen species is an inherent feature of PS II photochemistry. Significant knowledge gained over the past decade identifies individual steps involved in PS II repair. This progress has led to the formulation of a repair cycle that describes the life cycle of PS II, proceeding from its damage, disassembly, degradation, and resynthesis to its reassembly (2-4).To appreciate the challenges for repairing damaged PS II, it is essential to understand two structural features of the thylakoid membrane system. The first is that functional PS II is organized as a dimeric 1.4-MDa supercomplex in which each monomer consists of at least 28 subunits with two trimeric light harvesting complexes II (LHC II) attached to the core (5, 6). Within this huge supercomplex, the main target of photodamage is the central D1 subunit (7). The PS II repair cycle is therefore mainly designed for specific replacement of the damaged D1, which requires disassembly and reassembly of the whole supercomplex. The second characteri...

Singlet oxygen- and EXECUTER1-mediated signaling is initiated in grana margins and depends on the protease FtsH2

“…Damaged D1 is proteolytically cleaved by the FtsH protease (18,19). As most of the FtsH is found in grana margins (15,17), this part of the grana stacks seems to be a main site of protein degradation (15). This conclusion is in line with the high enrichment of FtsH2 in the 10,000 × g grana stack fraction and isolated margins and its low concentration in the grana core fractions ( Fig.…”

“…4 A, c). As shown above, FtsH2 colocalizes with EX1 in grana margins, and it is also involved in proteolytic degradation of D1 (15,17,18). In addition to FtsH, under high light stress Deg proteases such as Deg5 and Deg8 also contribute to the proteolytic breakdown of D1 (25).…”

“…To test this hypothesis, grana core and margins were separated and their polypeptide compositions were compared. The method for isolating grana margins (15) and separating them from grana cores is similar to the procedure used for the fractionation of thylakoid membranes (11,12), except that the incubation of isolated membranes occurs at a higher digitonin concentration and for a longer time. The results of this fractionation procedure confirmed our previous suggestion: The purified margins were highly enriched in CURT1A and EX1, whereas only trace amounts of these proteins were found in the grana core and stroma lamellae fractions (Fig.…”

“…Grana margins play an important role during the repair of damaged PSII complexes (15)(16)(17). The repair cycle is primarily designed for the specific replacement of D1, which requires the disassembly and reassembly of PSII complexes and depends on rapid protein translocation between grana core, grana margin, and stroma lamella (15). Damaged D1 is proteolytically cleaved by the FtsH protease (18,19).…”

Light-harvesting Complexes (LHCs) Cluster Spontaneously in Membrane Environment Leading to Shortening of Their Excited State Lifetimes

“…Interestingly, such a degree of curvature is similar to that of the grana margin of the thylakoid membrane (37). These regions are particularly important for the regulation of association of LHCII with the two photosystems (38) and for the disassembling/repairing of photosystem II (39). It is thus tempting to speculate that the membrane curvature helps in the dissociation of multiprotein complexes.…”