State Transitions also appear to be involved in turnover of the D1 protein of PSII, which is rapidly damaged in proportion to light intensity, even under low light conditions (Kim et al. The kinase is activated at the b6f complex by the binding of reduced plastoquinol to the complex, thereby linking phosphorylation of LHCII to the redox state of the plastoquinone pool (Shapiguzov et al. Phosphorylation of LHCII in State Transitions is catalyzed by the Stn7 kinase (Bellafiore et al. The re-allocation of the light-harvesting antenna to PSI also requires the minor PSI subunits PsaL and PsaH, which together form the docking site for LHCII (Lunde et al.
LHCII is phosphorylated during State Transitions, which involves the shifting of a portion of the major light-harvesting antenna from PSII to PSI (Larsson et al. The regulatory importance of the thylakoid dynamics, as implied above, is suggested by the observation that the degree of stacking of the thylakoids is highly variable, with phosphorylation of LHCII and PSII core components contributing to “unstacking” of the grana (Fristedt et al. The protein composition of these domains is not static, however, and lateral movement of photosynthetic complexes between the thylakoid compartments appears to be a central aspect of their function. Consequently, the various domains of the thylakoids can be defined by their specific protein components. These regions have been identified as the site of PSII degradation, and it is also the site at which both PSII and PSI can interact directly with each other (Tikkanen et al.
The degree of granal stacking appears to control, among other things, the edges of grana that form the interface between the granal stacks and the stromal lamellae. In addition, the newly discovered CURT proteins, which are responsible for the extreme curvature of the membranes at the grana margins, also appear to be necessary for the formation of granal stacks (Armbruster et al. The major light-harvesting antenna, LHCII, is known to be mainly responsible for stacking of the thylakoids, i.e., membranes devoid of other photosystem components still undergo extensive stacking, while membranes lacking LHCII do not stack (Mullet and Arntzen 1980 Day et al. Furthermore, stacking of the grana is variable, and the factors affecting this dynamic morphology are only now becoming better understood (Chow et al.
#Psi bands cvs full
2010) however, a full understanding of thylakoid compartmentalization is still lacking. As a result, separation of the photosystems is currently believed to be necessary to limit excitonic “spillover” from photosystem II to photosystem I (Haferkamp et al. Hence, lateral segregation is presumed to confer significant advantages to overcome this inefficiency. The sub-compartmentalization of the thylakoids by stacking of the grana comes with apparent kinetic costs, requiring diffusion of components (e.g., plastoquinone, plastocyanin, xanthophylls) to and from far-flung sites of activity in their various redox cycles (Trissl and Wilhelm 1993). Indeed, new components and previously underappreciated details of these dynamics are continually emerging.
The dynamics of the photosystem complexes, and that of the thylakoid structures in which they operate, are fundamental to the regulation of photosynthesis. Although the thylakoid membranes lend structure to the photosynthetic components, it is the protein complexes themselves which create and define the compartments in which they are embedded. This structuring of the thylakoids produces a compartmentalization of the components of the light reactions, with PSII and the major light-harvesting antenna confined to the appressed regions of the granal stacks, while the bulkier PSI and ATP synthase are excluded to the stromal lamellae and granal end membranes (Andersson and Anderson 1980 Pribil et al. The thylakoid membranes in higher plants are highly structured, and are traditionally described as being separated into two major components: stacks of disk-shaped membranes, the grana, which are connected laterally by sheets of stroma-exposed membrane, the stromal lamellae. The light reactions take place in pigmented protein complexes embedded in the thylakoid membranes of the chloroplast. Understanding how this is accomplished remains one of the most active areas of research in the field of photosynthesis, as it holds promise for needed improvements in both food and fuel production (Zhu et al. This regulation is crucial for efficient energy utilization and to avoid or mitigate photo-oxidative damage (Yamori 2016). The light reactions of photosynthesis are dynamically regulated in response to changing environmental conditions.
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