Structural insights into a dimeric Psb27-photosystem II complex from a cyanobacterium Thermosynechococcus vulcanus

Abstract

Photosystem II (PSII) is a multisubunit pigment-protein complex and catalyzes light-driven water oxidation, leading to the conversion of light energy into chemical energy and the release of molecular oxygen. Psb27 is a small thylakoid lumen-localized protein known to serve as an assembly factor for the biogenesis and repair of the PSII complex. The exact location and binding fashion of Psb27 in the intermediate PSII remain elusive. Here, we report the structure of a dimeric Psb27-PSII complex purified from a psbV deletion mutant (ΔPsbV) of the cyanobacterium Thermosynechococcus vulcanus, solved by cryo-electron microscopy. Our structure showed that Psb27 is associated with CP43 at the luminal side, with specific interactions formed between Helix 2 and Helix 3 of Psb27 and a loop region between Helix 3 and Helix 4 of CP43 (loop C) as well as the large, lumen-exposed and hydrophilic E-loop of CP43. The binding of Psb27 imposes some conflicts with the N-terminal region of PsbO and also induces some conformational changes in CP43, CP47, and D2. This makes PsbO unable to bind in the Psb27-PSII. Conformational changes also occurred in D1, PsbE, PsbF, and PsbZ; this, together with the conformational changes occurred in CP43, CP47, and D2, may prevent the binding of PsbU and induce dissociation of PsbJ. This structural information provides important insights into the regulation mechanism of Psb27 in the biogenesis and repair of PSII.

Properties of Photosystem II lacking the PsbJ subunit

“…In contrast, Psb27, Psb28 and Tsl0063, have been found to be associated to a proportion of the PsbJ-PsbA1/PSII (Nowaczyk et al 2012). These three proteins are known to be PSII assembly factors (Nowaczyk et al 2006;Roose and Pakrasi 2008;Komenda et al 2012;Liu et al 2013;Huang et al 2021;Zabret et al 2021). It should be noted that Tsl0063 is named either Psb34 in (Zabret et al 2021) or Psb36 in (Xiao et al 2021).…”

“…It should be noted that Tsl0063 is named either Psb34 in (Zabret et al 2021) or Psb36 in (Xiao et al 2021). Structures of PSII corresponding to assembly intermediates have been recently solved by using cryo-EM by using different strategies (Zabret et al 2021;Huang et al 2021;. Upon deletion of the psbJ gene it became possible to isolate a "PSII-I" intermediate with Psb27, Psb28 and Tsl0063 bound and in which the major conformational change was a distortion of the QB binding site and the replacement of bicarbonate with glutamate as a ligand of the non-heme iron (Zabret et al 2021).…”

“…Since the presence of Psb27 prevents the binding of PsbU and induces the dissociation of PsbJ and PsbY (Huang et al 2021) and since PsbJ also seems to trigger the release of Psb28 (Zabret et al 2021), the binding of PsbJ during the PSII assembly process very likely occurs after the release of Psb27 and Psb28. Consequently, a PSII without Psb27, Psb28, Tsl0063, and without PsbJ would logically also correspond to an assembly intermediate.…”

“…Upon deletion of the psbJ gene it became possible to isolate a "PSII-I" intermediate with Psb27, Psb28 and Tsl0063 bound and in which the major conformational change was a distortion of the QB binding site and the replacement of bicarbonate with glutamate as a ligand of the non-heme iron (Zabret et al 2021). From a psbV deletion mutant a dimeric Psb27-PSII (Huang et al 2021) and Psb28-PSII (Xiao et al 201) have also been purified in which the dissociation of PsbJ occurs. Such changes are proposed to protect the PSII from damage during biogenesis until an active Mn4CaO5 cluster is assembled (Zabret et al 2021;Xial et al 2021).…”

“…Other subunits bind to PSII during assembly, like Psb27, Psb28, Tsl0063. The presence of Psb27 has been proposed (Zabret et al 2021;Huang et al 2021;Xiao et al 2021) to prevent the binding of PsbJ, a single transmembrane -helix close to the quinone QB binding site. Consequently, a PSII rid of Psb27, Psb28 and Tsl0034 prior to the binding of PsbJ would logically correspond to an assembly intermediate.…”