The light-harvesting complex of photosystem II in barley. Structure and chlorophyll organization

Abstract

The isolated light-harvesting complex of photosystem II (LHCII) in barley had a chlorophyll (Chl)a/b ratio of 1.37 (mol/mol) and contained an average of 13.1 Chl molecules per 25 kD polypeptide chain, corresponding to 7.6 molecules of Chla and 5.5 molecules of Chlb. Tryptic digestion of native LHCII yielded a 16 kD fragment which contained 12 of these 13 Chl molecules. The 16 kD fragment started with the serine 52 of the mature polypeptide, as determined by N-terminal amino acid sequence analysis and comparison with the amino acid sequence deduced from the nucleotide sequence of a corresponding gene in wheat (Lamppa et al. 1985). Circular dichroism measurements of LHCII in 1% SDS or at extremes of pH, showed that there were at least two populations of Chla and that Chla was much more sensitive to changes in the aqueous environment than Chlb. We present a model for the folding of LHCII with four transmembarane helices, based on the above results on the application of the rules ofFasman (1978) andPaul andRosenbusch (1985) for the prediction of secondary structure using the wheat sequence.