Poster: Late and Moved Abstracts

Abs # 1357: Assembly of Light-Harvesting Complexes in Chlamydomonas reinhardtii

Presenter:	Eggink, Laura L., leggink@asu.edu
Authors	Eggink, Laura L. (A) (B)  Hoober, Kenneth  (A) (B)
Affiliations:	(A): Department of Plant Biology, Arizona State University (B): Center for the Early Events in Photosynthesis

Light-harvesting complexes (LHCs) are nuclear-encoded integral proteins in thylakoid membranes. Chlorophylls (Chl) a and b, along with active xanthophylls, are required co-factors in assembly and function of LHCs. Chl a and Chl b differ only in oxidation of the 7-methyl group of Chl a to a formyl group. Chlorophyll(ide) a oxygenase, the enzyme responsible for the oxidation, is membrane associated and located on the inner envelope and thylakoid membranes, as shown by western blotting and immunoelectron microscopy. Oxidation of the methyl group may be initiated by a radical mechanism, located on a conserved tyrosine, as shown by EPR. The ability of Chl b to form stronger coordination bonds with amino acid sidechains of the apoproteins of LHCs shifts equilibrium positions of complex formation toward assembly of LHCs in chloroplasts. Molecular modeling of a highly conserved amino acid region within the apoprotein of LHCs, Glu-X-X-His/Asn-X-Arg, suggested that this motif may bind two molecules of Chl. The prediction was confirmed experimentally, which suggests that this motif may bind Chl during import of LHCs. To indicate the importance of the sequence, it was designated a "retention motif". When Chl is not available during import, assembly does not proceed and LHC apoproteins are shunted to cytoplasmic vacuoles. During early development of the chloroplasts, the site of assembly of LHCs is on a low-density membrane (inner envelope), which vesiculates to form the thylakoid system.