PARENT SESSION
Posters P4Aa Chlorophyll and bilin based antenna systems. Abstracts (239-271)

LHCA5 - A novel light-harvesting protein: pigment binding, spectral properties and oligomerization behaviour. Stefanie Storf^*,1, Frank Klimmek², Stefan Jansson², Volkmar Schmid¹, ¹ Institut für Allgemeine Botanik, Mainz, Germany² Department of Plant Physiology, Umeå, Sweden

ABSTRACT- Photosystem I (PSI) of higher plants is a multi subunit complex that is composed of a central core complex harboring the reaction center and an outer light-harvesting antenna (LHCI) collecting solar radiation. For a long period, LHCI was thought to consist of four apoproteins (Lhca1 to Lhca4). However, genomic data from Arabidopsis thaliana later suggested the presence of two additional Lhca proteins belonging to PSI (Lhca5, Lhca6). Recently, production of antibodies raised against a peptide derived from the sequence of the lhca5 gene allowed detection of Lhca5 by immunoblotting in A. thaliana. Additionally, high-resolution SDS-PAGE in combination with tandem mass spectrometry led to the identification of peptides corresponding to the gene product of lhca5 in PSI preparations of tomato. These experiments demonstrated the expression of Lhca5 in higher plants for the first time. To test whether Lhca5 actually binds pigments, we constructed an expression vector with lhca5, overexpressed the protein and reconstituted it with total pigment extracts of thylakoids. Partially denaturing LDS-PAGE and sucrose density gradient centrifugation resulted in resolution of a monomeric pigment-protein complex confirming that Lhca5 is a light-harvesting protein. HPLC analyses of the recombinant LHC revealed binding of chlorophyll (Chl) a and b at a ratio of 2.5 and a Chl/carotenoid ratio of 4.5. The significance of individual pigments for LHC formation was analyzed by omitting individual pigments from the reconstitution mixtures. These experiments showed that all individual pigments are dispensable for reconstitution with the exception of Chl b. In addition we analyzed the spectral properties of reconstituted Lhca5 and studied the oligomerization behaviour of Lhca5. The latter was done by reconstitution of Lhca5 in the presence of each of the other Lhca proteins. The results obtained with Lhca5 will be presented and the data will be compared to those of reconstituted Lhca1 to Lhca4.

KEY WORDS: lhca, light harvesting complex, photosystem I, reconstitution