Modern Topics in the Phototrophic Prokaryotes by Patrick C. Hallenbeck

Author:Patrick C. Hallenbeck
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

The low number of observed connection sites (Fig. 12) is in clear contrast to the expected flux of material necessary for biogenesis and maintenance of the TM and therefore an active research field. In case of PS2, Fig. 13 illustrates the synthesis of non-active pre-complexes in the CM and their (potential) flow to the TM through thylakoid centers. Thylakoid centers are circular structures at the interface between thylakoid and cytoplasmic membrane , which seem to play a role in thylakoid membrane biogenesis, but also in the assembly process of PS2 in cyanobacteria (Nickelsen et al. 2013). PS2 biogenesis is a highly ordered process that is orchestrated by numerous additional proteins, the so-called PS2 assembly factors (Nixon et al. 2010). They guide the formation of preassembled PS2 modules that are formed by a subset of PS2 subunits at specific locations within the cellular membrane system, and they are also involved in merging these modules to distinct intermediates in the PS2 assembly process (Heinz et al. 2016). Briefly, the preassembled pD1-PsbI and D2-Cyt. b559 modules assemble in the thylakoid centers (TCs ) to the first reaction center (RC) complex, which is already capable of charge separation (Fig. 13). Here the D1 C-terminus is processed to the intermediate form by the D1 C-terminal processing peptidase (CtpA) which is an important control of the assembly process, as only after full processing of D1 the functional WOC can be formed. Another assembly factor, PratA (Klinkert et al. 2004), which is also involved in manganese delivery to PS2 (Stengel et al. 2012) is assisting this process. The complexes are then moved from the so-called pratA-defined membranes (PDMs), which are at least part of the TCs , to the thylakoid membrane . Here, CP47 assembles with the RC after full D1 processing, and the RC47 intermediate complex is formed (Boehm et al. 2012a). After subsequent attachment of the CP43-Psb27 module (Komenda et al. 2012a), the membrane intrinsic part of PS2 is fully assembled although still inactive in water oxidation. While WOC formation is facilitated by Psb27 (Nowaczyk et al. 2006; Roose and Pakrasi 2008; Cormann et al. 2016), the complex gains water-splitting activity after attachment of the extrinsic proteins PsbO, PsbU, and PsbV. Finally, two PS2 monomers assemble to one PS2 dimer and, dependent on the light conditions, PS2-dimer-phycobilisome supercomplexes are formed (Mullineaux 2008).

Fig. 13Spatial distribution of PS2 assembly. PS2 complexes are assembled via distinct intermediates with involvement of specialized membranes (PDMs) that are located at the interface between thylakoid membrane (TM ) and cytoplasmic membrane (CM ). For details, see text. PP periplasm, C cytoplasm, L lumen, RC reaction center complex

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