Unique organization of photosystem II supercomplexes and megacomplexes in Norway spruce

Abstract

Photosystem II (PSII) complexes are organized into large supercomplexes with variable amounts of light-harvesting proteins (Lhcb). A typical PSII supercomplex in plants is formed by four trimers of Lhcb proteins (LHCII trimers), which are bound to the PSII core dimer via monomeric antenna proteins. However, the architecture of PSII supercomplexes in Norway spruce[Picea abies (L.) Karst.] is different, most likely due to a lack of two Lhcb proteins, Lhcb6 and Lhcb3. Interestingly, the spruce PSII supercomplex shares similar structural features with its counterpart in the green alga Chlamydomonas reinhardtii [Kouřil et al. (2016) New Phytol. 210, 808-814]. Here we present a single-particle electron microscopy study of isolated PSII supercomplexes from Norway spruce that revealed binding of a variable amount of LHCII trimers to the PSII core dimer at positions that have never been observed in any other plant species so far. The largest spruce PSII supercomplex, which was found to bind eight LHCII trimers, is even larger than the current largest known PSII supercomplex from C. reinhardtii. We have also shown that the spruce PSII supercomplexes can form various types of PSII megacomplexes, which were also identified in intact grana membranes. Some of these large PSII supercomplexes and megacomplexes were identified also in Pinus sylvestris, another representative of the Pinaceae family. The structural variability and complexity of LHCII organization in Pinaceae seems to be related to the absence of Lhcb6 and Lhcb3 in this family, and may be beneficial for the optimization of light-harvesting under varying environmental conditions.

Keywords: Picea abies; Pinus sylvestris; clear native polyacrylamide electrophoresis; grana membrane; megacomplex; photosystem II; single-particle electron microscopy; supercomplex.

Figure 1

Separation of photosystem II (PSII) supercomplexes and megacomplexes from Norway spruce using clear native−polyacrylamide gel electrophoresis (CN−PAGE). Isolated thylakoid membranes were mildly solubilized by n‐dodecyl α‐d‐maltoside. The red and blue asterisks (the bands I and II) indicate the high‐molecular‐weight bands containing large PSII supercomplexes and megacomplexes, which were subjected to structural analysis by single‐particle electron microscopy. The bands of lower molecular weight represent different forms of PSII supercomplexes, PSI complex and PSII core complex, and LHCII proteins, respectively.

Figure 2

The large photosystem II (PSII) supercomplexes from Norway spruce. The supercomplexes were eluted from the band I (a–c, g, h) and the band II (d–f) in Figure 1. Projection maps of individual types of the PSII supercomplexes represent the best class averages of: (a) 12 015; (b) 9847; (c) 6356; (d) 622; (e) 1298; (f) 1018; (g) 1554; (h) 1219 particles. (i–p) Structural models of PSII supercomplexes were obtained by a fit of the high‐resolution structure (van Bezouwen et al., 2017). Individual PSII subunits are color‐coded: dark green – core complex; yellow – S trimer; cyan – M trimer; red – N trimer; green – L trimer; magenta – Lhcb5; orange – Lhcb4.

Figure 3

Various photosystem II (PSII) megacomplexes from Norway spruce. The megacomplexes were eluted from the band II (Figure 1). Projection maps of individual types of the PSII megacomplexes represent the best class averages of: (a) 1549; (b) 3118; (c) 1095; (d) 724; (e) 1178; (f) 915; (g) 1103 particles. (h–n) Structural models of the PSII megacomplexes obtained by a fit of the high‐resolution structure of PSII (van Bezouwen et al., 2017). Individual PSII subunits are color‐coded: dark green – core complex; yellow – S trimer; cyan – M trimer; green – L trimer; magenta – Lhcb5; orange – Lhcb4.

Figure 4

Distribution of photosystem II (PSII) complexes and their association into megacomplexes in isolated grana membranes. (a) An example of the electron micrograph of negatively stained grana membranes isolated from Norway spruce showing a density and distribution of PSII complexes. White arrow indicates a typical density of the PSII core complex. (b) Most abundant associations of PSII complexes into different types of megacomplexes found within the grana membranes after image analysis. (c) Structural models of the PSII megacomplexes obtained by a fit of the high‐resolution structure (van Bezouwen et al., 2017). Individual PSII subunits are color‐coded: dark green – core complex; yellow – S trimer; cyan – M trimer; magenta – Lhcb5; orange – Lhcb4.

Figure 5

A hypothetical model of photosystem II (PSII) supercomplex from Norway spruce (Picea abies) and its comparison with evolutionary different organisms. The model of the complete PSII supercomplex in Norway spruce is based on the structures of different forms of PSII supercomplexes revealed by single‐particle electron microscopy. The specific orientation of the N trimer in Norway spruce probably enables a stable binding of the L trimer and formation of the second row of additional L_a trimers along the PSII core complex, but still keeps the path for plastoquinone molecules free. Different orientation of the N trimer in Chlamydomonas reinhardtii (Shen et al., 2019) likely does not support the binding of the L trimer. In the majority of land plants, the binding site for the N trimer is occupied by the Lhcb6 protein, which probably modifies the binding of the M trimer. The L trimer can very occasionally bind to the PSII core complex (e.g. in spinach; Boekema et al., 1999a,b) or additional L_a trimers can associate with PSII at the site of the S/M trimers (e.g. Arabidopsis; Nosek et al., 2017). Individual PSII subunits are color‐coded: dark green – core complex; yellow – S trimer; cyan – M trimer; red – N trimer; green – L trimer; light green – L_a trimers; magenta – Lhcb5; orange – Lhcb4.