The preparation and characterisation of native photoreceptor units from the thylakoids of Rhodopseudomonas viridis

Abstract

The photosynthetic membranes of Rhodopseudomonas viridis consist of a regular array of structural units. Each unit is composed of a central core (thought to contain the reaction centre complex) surrounded by a subdivided ring of protein (of likely antennae function). These individual units can be dissociated from the membrances using a variety of detergent treatments. The absorption spectrum, used as a criterion of a native state, is retained. All of the seven major polypeptides, the four reaction centre polypeptides (cytochrome, H, M and L chain) as well as the three light-harvesting polypeptides (B1015-alpha, beta and xi) are shown to be present. Electron microscopy of the units shows a similar structure to the units within the membrane. surface-specific iodination of both membranes and units labels predominantly polypeptides H, B1015-alpha, and xi. M and L are weakly labelled. In addition, B1015-beta is labelled in the isolated units. This, with other evidence, supports an allocation of light-harvesting polypeptides to the outer ring. Further solubilisation of these units separates the reaction centre (as a native complex containing all four polypeptides) from the light-harvesting polypeptides. The light-harvesting polypeptides are obtained in a form containing all three polypeptides and bound pigment, however the peak at 1015 nm corresponding to native bacteriochlorophyll b is lost.