Cloning and Characterization of a Cdna for Algal Homologue of CP26 in the Light-Harvesting Complex II of Chlamydomonas Reinhardtii

Abstract

CP29, CP26, and CP24 have been identified as ‘minor’ Chl-alb binding proteins, since they were less abundant and copurified in the ‘LHC II-fraction’. Due to the low amount of bound pigments, it seems unlikely that the main function of these Chl-alb binding proteins is to harvest light. In fact, important observations have been reported in the past decade, suggesting that these proteins play crucial role(s) in energy distribution and photoprotection in green plants. These observations include, 1) non-photochemical quenching is closely related to the xanthophyll cycle turnover, 2) q_E-quenching is inhibited by DCCD, 3) DCCD is preferentially bound to CP26 and CP29, 4) xanthophyll violaxanthins are preferentially bound in these minor Chl-alb binding proteins, and 5) the minor Chl-alb binding proteins are localized between the outer antenna and the reaction center core (for review, see ref. 1, 2). The unicellular green alga Chlamydomonas reinhardtii provides an excellent system to study the photosynthetic apparatus because of the ability to grow heterotrophically, the feasibility of performing biophysical measurements in vivo and availability of well-established techniques for transforming cells with foreign DNA. Genes encoding CP26 have been previously characterized from scots pine (3), tomato (4), barley (5), and maize (6). However, whereas homologues of CP26 and CP29 in this alga, p10 and p9, respectively, had been identified (7), there has been no report on their polypeptide sequences. In this study, p 10 protein was purified from C. reinhardtii and the partial amino acid sequences were determined, we further report the isolation and characterization of a cDNA encoding p10 protein with the aim of understanding to what extent they are comparable to those in higher plants and, in the long run, of investigating physiological roles of the polypeptides in vivo.