The membrane-intrinsic light-harvesting complex of the red alga Galdieria sulphuraria (formerly Cyanidium caldarium): Biochemical and immunochemical characterization

The membrane-intrinsic light-harvesting complex of the red alga Galdieria sulphuraria (formerly Cyanidium caldarium) could be isolated by gel-electrophoresis as a green band with an apparent molecular mass of about 20 kDa. The band had a long-wavelength absorption maximum at 672 nm and a fluorescence maximum (77 K) at 680 nm and reacted with an antibody against light-harvesting proteins of higher plant Photosystem I. Screening of thylakoid membranes with antisera directed against various chlorophyll a/b and chlorophyll a/c light-harvesting proteins indicated the existence of at least 4 distinct light-harvesting polypeptides with apparent molecular masses between 17 and 20 kDa. Isolation of Photosystem I and of a fraction enriched in Photosystem II showed that these polypeptides are exclusively bound to Photosystem I, thus forming a holocomplex which binds at least 205 molecules of chlorophyll a, and 33 and 37 molecules of zeaxanthin and beta-carotene, respectively. Additionally, there is some evidence for the existence of a second Photosystem I pool without light-harvesting complexes. In-vitro translation experiments showed that at least two of the five polypeptides which constitute the membrane-intrinsic light-harvesting complex of Galdieria sulphuraria are translated from the poly(A)-enriched RNA fraction. They could be immunoprecipitated as preproteins being 3 to 4 kDa larger in size than the mature polypeptides.