Comparative analysis by differential RNA display (DDRT-PCR) of the expression patterns of potato plants induced and non-induced to tuberize, led to the isolation of a cDNA clone, C40.4, that is strongly upregulated in the leaves of tuberizing plants. Leaf expression of this transcript was shown to be light-dependent, with increased levels of mRNA and protein being detected during the light hours. Sequence analysis revealed near complete identity to potato CDSP34, a thylakoid protein induced by drought stress, and strong homology to the carotenoid-associated proteins fibrillin, ChrB and PAP from pepper fruit, and CHRC from cucumber flowers. By using an antibody against pepper fibrillin, we were able to demonstrate a chloroplastic location of the C40.4 protein in association with the thylakoid membranes. Two-dimensional analysis of thylakoid pigment-protein complexes showed a specific association of the protein with the photosystem II (PSII) multi-subunit complex. Antisense plants with reduced levels of accumulation of C40.4 showed a stunted growth and decreased tuber yield and exhibited reduced values of non-photochemical quenching of chlorophyll a fluorescence. Altogether, these results indicate a preferential association of the C40.4 protein with the light harvesting complex of photosystem II (LHCII) antenna pigment complexes, and suggest a functional role of this protein in photosynthesis, by modulating photosynthetic efficiency and dissipation of excess absorbed light energy within the antenna complex.