PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

Interaction studies of chlorosome proteins and characterization of mutants lacking [2Fe-2S] chlorosome proteins in Chlorobium tepidum. Hui Li^*,1, Niels-Ulrik Frigaard¹, Donald Bryant¹, ¹ 232 South Frear, University Park, Pennsylvania, USA

ABSTRACT- In Chlorobium tepidum, ten kinds of proteins (CsmA, CsmB, CsmC, CsmD, CsmE, CsmF, CsmH, CsmI, CsmJ and CsmX) are embedded on the monolayer chlorosome membrane. Wild type chlorosomes and chlorosomes from mutants lacking a single kind of protein were cross-linked with zero-length cross-linker 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), analyzed by SDS-PAGE and detected by immunoblotting with polyclonal antibodies against chlorosome proteins. CsmA, the dominating component of the baseplate, was found to form dimer, trimer, and other multimers up to octamer by itself in both wild type and SDS-treated chlorosomes. CsmA multimers were also detected in carotenosomes, the vestigial chlorosomes in bchK knock-out mutant. Cross-linking between CsmA and its precursor (pre-CsmA), between pre-CsmA and CsmB or CsmF were also detected, which indicates that pre-CsmA might be located on the edge of the CsmA baseplate, and the interactions between pre-CsmA and CsmB/CsmF only happen on the baseplate edge. CsmC is another chlorosome protein that forms homo-multimers. Iron-sulfur chlorosome proteins CsmI and CsmJ form heterodimers and both of them interact with CsmB. Based on the information obtained by cross-linking experiments, a model about protein organization and interaction on the chlorosome membrane is drawn. Three chlorosome proteins, CsmI, CsmJ, and CsmX have shown strong sequence similarity in their amino-terminal domains to [2Fe-2S] ferredoxins of the adrenodoxin/putidaredoxin subfamily. Insertional inactivation of the three iron-sulfur proteins has been made in all combinations. Comparison of wild type and the mutated chlorosomes in fluorescence quenching/restoration strongly suggests that the iron-sulfur proteins, especially CsmI and CsmJ, play important roles in the redox regulation of energy transfer within the chlorosome. Under oxygen stress conditions, CsmI/J might transfer electrons from the quencher within the chlorosome (most probably chlorobiumquinone) to molecular oxygen to active the quencher and reduce the energy transfer from BChl c rods to reaction centers. Under oxygen release conditions, CsmI/J might transfer electrons in the other direction to inactive the quencher and restore the energy transfer. Similar quenching/restoration mechanisms also exist in whole cells.

KEY WORDS: iron-sulfur chlorosome proteins, Interaction of chlorosome proteins, cross-linking, redox regulation of energy transfer