CELLULAR ARCHITECTURE III: LIGHT HARVESTING COMPLEX II

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This project deals with a problem of fundamental importance in living cells, membranemorphogenesis. The project focuses, however, on a non-medical organism,photosyntetic bacteria; their chromatophores offer a prime example of membranemorphogenisis. The chromatophores of purple photosynthetic bacteria appear to beformed by the aggregation and self-organization of the photosynthetic proteins inthe membrane [1, 2]. The overall shape of the chromatophore varies among speciesand with protein composition and depends on the arrangement of light-harvestingcomplex II (LH2) and light-harvesting complex I (LH1). A combination of LH2sand dimeric LH1s results in a spherical chromatophore, as does the presence ofLH2 by itself [3,4]. Lamellar chromatophores generally contain LH2s together withmonomeric LH1s [5,6]. We are interested in exploring how LH2 produces curvature,both by itself and in combination with LH1.We found previously that hexagonally-packed LH2s in simulation equilibrate toform a curved protein patch [7]. The extent of the curvature was dependent onhow closely the proteins were packed in the membrane and which bacterial speciesthe LH2 crystal structure or model was from. All of the LH2 systems formedcurvature, even those from species with naturally lamellar, i.e. flat, chromatophores,suggesting that the formation of spherical curvature via aggregation is common toall LH2s [7].Our recent work aims to expand our understanding of the mechanism by which LH2-LH2 interactions produce curvature. It appears that each LH2 in the aggregate isinclined to tilt away from its neighbors due to a combination of steric interactionsand the electrostatic repulsion of conserved charged residues on the cytoplasmic sideof the proteins. Modified LH2s in which these residues were replaced with neutralresidues produced less curvature than their unmodified counterparts. We also foundthat LH2s packed around an LH1 monomer produced almost no curvature over thesame simulation timescale of the LH2-only systems. This seems to be due in partto a mismatch in the placement of the charged residues on LH1 vs. LH2, andis consistent with the experimental observation that flat chromatophores containmostly a homogeneous mixture of LH2s and LH1 monomers. These results havebeen submitted for publication. In the next year, we plan to expand our simulationsto include more LH2s (as experiments suggest that higher concentrations of LH2lead to greater curvature [4]) and to cover larger areas of mixed LH1/LH2 regions asseen in AFM images of chromatophores to better understand the interplay betweenthe two proteins.BIBLIOGRAPHY[1] C. N. Hunter, J. D. Tucker, and R. A. Niederman. The assembly and organisation ofphotosynthetic membranes in Rhodobacter sphaeroides. Photochem. Photobiol. Sci.,4:10231027, 2005.[2] R. N. Frese, J. C. P`amies, J. D. Olsen, S. Bahatyrova, C. D. van der Weij-de Wit,T. J. Aartsma, C. Otto, C. N. Hunter, D. Frenkel, and R. van Grondelle. Proteinshape and crowding drive domain formation and curvature in biological membranes.Biophys. J., 94:640647, 2008.[3] S. Bahatyrova, R. N. Frese, C. A. Siebert, J. D. Olsen, K. O. van der Werf, R. vanGrondelle, R. A. Niederman, P. A. Bullough, C. Otto, and C. N. Hunter. The nativearchitecture of a photosynthetic membrane. Nature, 430:10581062, 2004.[4] J. N. Sturgis and R. A. Niederman. The effect of different levels of the B800-B850light-harvesting complex on intracytoplasmic membrane development in Rhodobactersphaeroides. Arch. Microbiol., 165:235242, 1996.[5] S. Scheuring, D. Levy, and J.-L. Rigaud. Watching the components of photosyntheticbacterial membranes and their in situ organisation by atomic force microscopy.Biochim. Biophys. Acta, 1712:109127, 2005.[6] R. P. Gon¿calves, A. Bernadac, J. N. Sturgis, and S. Scheuring. Architecture of thenative photosynthetic apparatus of Phaeopirillum molischianum. J. Struct. Biol.,152:221228, 2005.[7] D. Chandler, J. Hsin, C. B. Harrison, J. Gumbart, and K. Schulten. Intrinsic curvatureproperties of photosynthetic proteins in chromatophores. Biophys. J., 95:28222836,2008.