Dynamics and diffusion in photosynthetic membranes from Rhodospirillum photometricum

¹ Institut Curie
² LISM/IBSM

^* To whom correspondence should be addressed. E-mail: sturgis{at}ibsm.cnrs-mrs.fr.

Submitted on February 22, 2006
Revised on March 22, 2006
Accepted on 11 August 2006

	Abstract

Photosynthetic organisms drive their metabolism by converting light energy into an electrochemical gradient with high efficiency. This conversion depends on the diffusion of quinones within the membrane. In purple photosynthetic bacteria quinones reduced by the reaction center (RC) diffuse to the cytochrome bc₁ complex, and then return once re-oxidized to the RC. In Rhodospirillum (Rsp.) photometricum the RC containing core-complexes are found in a disordered molecular environment, with fixed light-harvesting complex to core-complex ratio but without a fixed architecture, while additional light-harvesting complexes synthesized under low-light conditions pack into large para-crystalline antenna domains. Here, we have analyzed, using time-lapse atomic force microscopy (AFM), the dynamics of the protein complexes in the different membrane domains, and find that the disordered regions are dynamic while ordered antennae domains are static. Based on our observations we propose, and analyze using Monte-Carlo simulations, a model for quinone diffusion in photosynthetic membranes. We show that the formation of large static antennae domains may represent a strategy for increasing electron transfer rates between distant complexes within the membrane, and thus be important for photosynthetic efficiency

Key Words: AFM, LH2, core complex, membrane protein, monte carlo simulation, quinone