PARENT SESSION
Posters P8A Cytochrome b-c complexes. Abstracts (382-393)

Molecular basis of the major difference between the X-ray structure of cytochrome b₆f complex from M. laminosus and C. reinhardtii: Binding site of tridecyl-stigmatellin. Jiusheng Yan¹, William Cramer^*,1, ¹ Department of Biological Sciences, Purdue University, 915 West State Street, West lafayette, Indiana, USA

ABSTRACT- The 3.0-3.1 X-ray structures of the ctochrome b₆f complex from M. laminosus (ML) and C. reinhardtii(CR)in the presence of the p-side quinine-analogue inhibitor tridecyl-stigmatellin (TDS) are very similar. The major difference occurs in the p-side binding position of TDS and the resulting slight shift in position of the ISP soluble domain. In CR, TDS binds in a "head-in" mode, as found in the structures of the cytochrome bc₁ complex, with the ring moiety of the TDS proximal to the ISP [2Fe-2S] cluster and the 13 carbon tail extended to the portal that connects the large quinone exchange cavity and Q_p niche; in ML, TDS binds in "head-out" mode, with tail inserted in the opposite direction into portal and the ring is then caught in the quinone exchange cavity and is 20 away from the [2Fe-2S] cluster. Sequence and structure comparisons indicate that difference in residue 81 (CR-Phe/ ML-Leu) of subunit IV and 186 (CR-Leu/ ML-Ala) of cytocrhome b₆ may account for the observed difference in TDS binding in the two organisms. Using the transformable cyanobacterium Synechococcus sp. PCC 7002 as a model, it was observed that the sensitivity of electron transport in the b₆f complex to TDS and stigmatellin is: (a) approximately 10-fold enhanced upon mutational change of Leu-81 in subunit IV to Phe; (b) not affected by substitution of Leu 193 (equal to position 186 in CR and ML) in cytochrome b₆ by Ala; (c) almost completely eliminated by replacement of Leu111 in ISP (near [2Fe-2S] cluster) by Ala or Tyr. It is inferred that only the "head-in" binding mode is inhibitory. The difference in residue 81 of subunit IV may control the binding affinity of the TDS in two different binding modes in cyanobacteria, algae and higher plants. (supported by NIH GM-38323)

KEY WORDS: binding mode, stigmatellin, cytochrome b6f, Synechococcus sp. PCC 7002