HYPOTHESIS FOR THE EVOLUTION OF 3-HELIX CHL A B AND CHL A/C LIGHT-HARVESTING ANTENNA PROTEINS FROM 2-HELIX AND 4-HELIX ANCESTORS/

The nuclear-encoded Chi nib and Chi ale antenna proteins of photosynth etic eukaryotes are part of an extended family of proteins that also i ncludes the early light-induced proteins (ELIPs) and the 22 kDa intrin sic protein of PSII (encoded by psbS gene). All members of this family have three transmembrane helices except for the psbS protein, which h as four. The amino acid sequences of these proteins are compared and r elated to the three-dimensional structure of pea LHC II Type I (Kuhlbr andt and Wang, Nature 350: 130-134, 1991). The similarity of psbS to t he three-helix members of the family suggests that the latter arose fr om a four-helix ancestor that lost its C-terminal helix by deletion. S trong internal similarity between the two halves of the psbS protein s uggests that it in turn arose as the result of the duplication of a ge ne encoding a two-helix protein. Since psbS is reported to be present in at least one cyanobacterium, the ancestral four-helix protein may h ave been present prior to the endosymbiotic event or events that gave rise to the photosynthetic eukaryotes. The Chi alb and Chi ale antenna proteins, and the immunologically-related proteins in the rhodophytes may have had a common ancestor which was present in the early photosy nthetic eukaryotes, and predated their division into rhodophyte, chrom ophyte and chlorophyte lineages. The LHC I-LHC II divergence probably occurred before the separation of higher plants from chlorophyte algae and euglenophytes, and the different Types of LHC I and LHC II protei ns arose prior to the separation of angiosperms and gymnosperms.