ISOLATION AND CHARACTERIZATION OF A PROTEIN ASSOCIATED WITH CAROTENE GLOBULES IN THE ALGA DUNALIELLA BARDAWIL

The halotolerant alga Dunaliella bardawil accumulates very large amounts of beta-carotene when exposed to high light intensity. The accumulated beta-carotene is concentrated in small, oily globules within the chloroplast and has been suggested to protect the alga against photodamage by high irradiation (A. Ben-Amotz, A. Katz, M. Avron [1982] J Phycol 18: 529-537; A. Ben-Amotz, M. Avron [1983] Plant Physiol 72: 593-597; A. Ben-Amotz, A. Shaish, M. Avron [1989] Plant Physiol 91: 1040-1043). A 38-kD protein was identified and purified from beta-carotene globules and was designated carotene globule protein (Cgp). Induction of Cgp occurs in parallel with beta-carotene accumulation in D. bardawil grown under different inductive conditions. Cgp is overproduced in a constitutive mutant strain that overproduces p-carotene and is not detected in Dunaliella salina, a species that does not accumulate beta-carotene. Cgp production was not suppressed by norflurazon, an inhibitor of beta-carotene synthesis that leads to accumulation of the carotenoid precursor phytoene. Immunogold-labeling analysis by electron microscopy demonstrates that the protein is localized at the periphery of the globules. Proteolytic cleavage by trypsin enhances the coalescence and destruction of the globules, in parallel with Cgp disappearance. It is suggested that the function of Cgp is to stabilize the structure of the globules within the chloroplast.