Pigment-binding properties of the recombinant photosystem II subunit CP26 reconstituted in vitro

CP26 is the most recently described antenna protein in higher plants which has been reported to be involved in xanthophyll-dependent regulation of the light-harvesting function but is largely unknown due to the difficulties of purification. In this study we have overexpressed in Escherichia coli the Lhcb5 gene product and reconstituted CP26 in vitro by refolding the recombinant protein in the presence of chlorophyll a, chlorophyll b and xanthophylls. The resulting pigment-protein complex is stable enough to be isolated by partially denaturing gel electrophoresis. Reconstitution and isolation conditions for CP26 are similar to those used for other chlorophyll alb complexes like the major light-harvesting complex of photosystem II (LHCII) and CP29; however, CP26 differs with regard to its lower specificity in carotenoid binding. Most significantly, rather stable recombinant CP26 can be reconstituted containing violaxanthin as the only carotenoid. This enhanced plasticity with respect to carotenoid binding is consistent with CP26 being the major binding protein of violaxanthin involved in the xanthophyll cycle. The availability of recombinant CP26 opens the way to a better characterisation of this pigment-protein complex with regard to its biochemistry and its physiological functions.