Pigment-binding properties of mutant light-harvesting chlorophyll-a/b-binding protein

Light-harvesting chlorophyll-a/b-binding protein (LHCP), overexpressed in Escherichia coli, can be reconstituted with pigments to yield complexes that are structurally very similar to light-harvesting complex II (LHCII) isolated from thylakoids [Paulsen, H., Rümler, U. & Rüdiger, W. (1990) Planta 181, 204-211]. In order to analyze which domains of the protein are involved in pigment binding, we reconstituted deletion mutants of LHCP with pigments and characterized the resulting complexes regarding their pigment composition and spectroscopic properties. Series of progressive deletions from either end of the protein revealed that most of the N-terminal and part of the C-terminal hydrophilic regions of LHCP are dispensible for pigment binding. In either deletion series, the deletions that completely abolished reconstitution could be narrowed down to segments of five amino acids that do not contain histidine, asparagine, or glutamine. All mutants either formed complexes with both pigment composition and spectroscopic properties very similar to those of light-harvesting complex II isolated from thylakoids, or they did not form any stable complexes at all. There is no indication of a segment of LHCP binding a subset of LHCII pigments. We conclude that the stabilization of LHCP-pigment complexes is highly synergetic rather than based on individual pigment-binding sites provided by the protein.