Characterization of New Type II Water-Soluble Chlorophyll-Binding Proteins (WSCPs) from Brassicaceae

In photosynthetic bacteria and plants, chlorophyll (chl) molecules play vital roles in light-harvesting and charge separation. In the photosynthetic membrane, multiple chlorophylls form complexes with specific chl-binding transmembrane proteins that tune their light absorption properties to fit their specific roles. The water-soluble chlorophyll binding proteins from Brassicaceae (WSCPs) are a unique class of chlorophyll binding protein. These are stress inducible protein complexes, exhibiting high affinity for chls but not involved in the primary photosynthetic processes. The WSCP sub class II have attracted much attention as a model system for understanding spectral tuning in protein-pigment complexes and as novel templates for artificial protein-based light-harvesting systems. To date, only six class II WSCPs have been characterized, five of which were classified as type IIa and one as type IIb according to their distinct red light absorption bands at 673 nm, and 664 nm, respectively. Recently, we elucidated the molecular origin of the spectral differences between type IIa and type IIb WSCPs (Bednarczyk et al. Angew. Chemie. 2016). With this structural information, we carried out a phylogenetic analysis of identified and putative type II WSCPs across the Brassicaceae family. This was combined with biochemical and spectroscopic characterization of selected WSCPs expressed in E. coli and assembled with chls in vitro. We identified new members of the type II WSCP class: two of type IIb and two of type IIa. Intriguingly, we found an additional class of WSCPs that are incapable of chl binding. Extending the collection of characterized chl-protein complexes sheds new light on the factors affecting chl binding and spectral tuning in WSCPs.