Functional studies on the small Cab-like proteins of Synechocystis sp. PCC 6803

Abstract

In contrast to higher plants, cyanobacteria appear to lack chlorophyll b as well as multi-helix Cab proteins. The major peripheral light-harvesting complex in cyanobacteria is the phycobilisome, which is associated with the thylakoid membrane. But recently a small protein of 8 kDa was identified in the cyanobacterium Synechococcus sp. PCC 7942. This protein is predicted to have a single membrane-spanning helix, which shows significant sequence similarity to the first and third membrane-spanning region of proteins belonging to the cab family (Dolganov et al., Proc. Natl. Sci. USA 92, 636-640, 1995). By homologous searching, five genes have been identified in cyanobacterium Synechocystis PCC 6803, which are named as small cab like proteins (Scps). In view of their primary structure, these Scps may be involved in pigment binding (Funk and Vermaas, Biochemistry, 38, 9397-9404, 1999). In vitro reconstitution methods involved in overexpression of these proteins in E. coli and reconstituting the proteins with pigments extracted from cyanobacteria are adopted to test if these proteins bind pigment. In vivo overexpression of these proteins with His tag in cyanobacteria and then isolation of these proteins are also under way. Thus, we are allowed to investigate if the Scps are really involved in pigment binding and indeed a so far not detected family of additional antenna proteins in cyanobacteria. As different Scp mutants are available, we will also be able to find the conditions under which these proteins bind pigment.