title: Two-dimensional structure of the light-harvesting chlorophyll a/b complex by cryoelectron microscopy.
abstract: The light-harvesting chlorophyll a/b complex (LHC-II) found in green plants has at least three functions: it absorbs light energy for transfer to the reaction centers, it is involved in keeping the photosynthetic membranes stacked, and it regulates energy distribution between the two photosystems. We have developed a procedure to produce large vesicles consisting almost exclusively of two-dimensional crystalline domains of LHC-II in which LHC-II is biochemically and structurally intact, as shown by SDS-PAGE, response to cations, and 77K fluorescence excitation spectra. The vesicles were examined by cryoelectron microscopy and analyzed, in projection, to a resolution of 17 A. Their surface lattice consists of trimers arranged in interlocking circles; the two-sided plane group is p321 (unit cell dimension, a = 124 A) with two, oppositely facing trimers/unit cell. Individual trimers consist of matter arranged in a ring, around a central cavity, an appearance similar to that obtained in some conditions using negative stain (Li, J., 1985. Proc. Natl. Acad. Sci. USA. 82:386-390). The monomer (approximately 45 x 20 A) is seen as two domains of slightly different size at this resolution. The thickness of single layers is approximately 48 A, measured from edge-on views of the frozen vesicles. Based on these dimensions, the molecular mass of the monomer is approximately 30 kD. Therefore, each monomer appears to be composed of a single polypeptide and its associated pigments.
mesh_terms: D002734:Chlorophyll / Q000032:analysis* / Q000302:isolation & purification; D003460:Crystallization; D004591:Electrophoresis, Polyacrylamide Gel; D005615:Freezing; D007091:Image Processing, Computer-Assisted; D045342:Light-Harvesting Protein Complexes; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D045322:Photosynthetic Reaction Center Complex Proteins; D010940:Plant Proteins / Q000032:analysis* / Q000302:isolation & purification; D013050:Spectrometry, Fluorescence; D013194:Staining and Labeling
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D010940:Plant Proteins; D002734:Chlorophyll