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. The photosynthetic proteins in purple bacteria not only carry out the intricate processof energy conversion, but are also responsible for organizing the membrane intodistinct cellular compartments with well-defined shapes. Indeed, electron tomographyand electron microscopy have discovered that the photosynthetic proteins inpurple bacteria aggregate in the membrane to form independent photosyntheticunits with different shapes and sizes depending on species and protein composition.Among the membrane-bending photosynthetic proteins, the Rhodobactersphaeroides core complex is the only one thought to induce cylindrical curvature andbuild tubular vesicles in bacterial cells. However, lack of high resolution structuresfor the core complex has rendered it difficult to investigate its membrane-bendingmechanism. This project deals with a non-medical photosynthetic organism becauseof the principle importance of membrane morphogenesis for the cells of allorganisms.Previously, we constructed a rudimentary all-atom model for the Rhodobactersphaeroides core complex [1] based on the then-available two-dimensional electronmicroscope projection map [2], and showed that the core complex, a dimeric construct,bends slightly and produces curvature in the surrounding membrane. Althoughthese simulations explain the mechanism of core complex-induced membranecurvature, the curvature observed was insufficient to reproduce the known size ofthe core complex tubular vesicles due to uncertainty of the core complex structure.Recently, a three-dimensional electron miscroscope map became available,displaying a highly-bent core complex [3] and provided an opportunity to furtherfine-tune our understanding of the core complex structure. Combining the earlierall-atom model with the new three-dimensional density map [3] using the moleculardynamics flexible fitting method [4], an improved core complex model was generated[5, 6]. The large bending of the complex induced a high local curvature in themembrane, which agreed well with the size of the core complex tubular vesicles [5].Furthermore, the simulations demonstrated how the local curvature properties ofthe RC-LH1-PufX dimer propagate to form the observed long-range organizationof the Rhodobacter sphaeroides tubular vesicles [5].BIBLIOGRAPHY[1] D. Chandler, J. Hsin, C. B. Harrison, J. Gumbart, and K. Schulten. Intrinsic curvatureproperties of photosynthetic proteins in chromatophores. Biophys. J., 95:2822?2836,2008.[2] P. Qian, C. N. Hunter, and P. A. Bullough. The 8.5 ?A projection structure of the coreRC-LH1-PufX dimer of Rhodobacter sphaeroides. J. Mol. Biol., 349:948?960, 2005.[3] P. Qian, P. A. Bullough, and C. N. Hunter. Three-dimensional reconstructionof a membrane-bending complex: The RC-LH1-PufX core dimer of Rhodobactersphaeroides. J. Biol. Chem., 283:14002?14011, 2008.[4] L. G. Trabuco, E. Villa, K. Mitra, J. Frank, and K. Schulten. Flexible fitting ofatomic structures into electron microscopy maps using molecular dynamics. Structure,16:673?683, 2008. PMCID: PMC2430731.[5] J. Hsin, J. Gumbart, L. G. Trabuco, E. Villa, P. Qian, C. N. Hunter, and K. Schulten.Protein-induced membrane curvature investigated through molecular dynamicsflexible fitting. Biophys. J., 2009. In press.[6] M. K. Sener, J. Hsin, L. G. Trabuco, E. Villa, P. Qian, C. N. Hunter, and K. Schulten.Structural model and excitonic properties of the dimeric RC-LH1-PufX complex fromRhodobacter sphaeroides. Chem. Phys., 357:188?197, 2009.
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