Spectroscopic characterization of CP26, a chlorophyll a/b binding protein of the higher plant Photosystem II complex

A spectroscopic study is presented of the minor chlorophyll a/b binding protein CP26 isolated from spinach by means of a dodecylmaltoside/betaine washing procedure. The preparations are characterized by a chlorophyll a/chlorophyll b ratio of 3.3 +/- 0.1, and most likely contain 2 chlorophyll b (Chl b) and 6-7 chlorophyll a (Chl a) molecules per monomeric protein. Some of the spectroscopic properties of CP26 show strong similarities to those of the major chlorophyll a/b light-harvesting protein LHC-II, which is in line with the sequence homologies between the two proteins. Spectroscopic differences in the Chl b absorption region are caused by the variation in Chl b content in both proteins. A strongly blue-shifted Chl b band at 637 nm in CP26 has similar linear and circular dichroism properties as the spectral component at 640 nm of LHC-II. It is suggested that these spectral features arise from a conserved Chl b molecule, and that the blue shifts are caused by charged amino acids in the vicinity of these Chl b molecules. The other Chl b band in CP26 is observed at 650 nm. Differences in the Chl a absorption region mainly concern the reduced absorption at 670 nm for CP26, whereas a strong band near 675 nm is very similar to the band at 676 nm for LHC II. Tentative assignments of several absorption bands of CP26 and LHC II to specific pigments are made on the basis of the recently reported three-dimensional structure of LHC II and of the primary amino acid sequences of both proteins. In the carotenoid region LHC II and CP26 show slightly different linear and circular dichroism signals. However, the differences are not large enough to exclude a similar arrangement of the two lutein molecules in LHC II and CP26.