X-射線結(jié)晶學(xué)能夠非常細(xì)致地反映呼吸和光合作用膜中的蛋白復(fù)合體的結(jié)構(gòu)?，F(xiàn)在,，AFM（原子力顯微鏡）已發(fā)展到有可能用來檢查X-射線結(jié)構(gòu)是否真實(shí)反映一個(gè)自然膜中的蛋白情況的程度，并已開始用來研究蛋白在活體中是怎樣相互作用的,。本期Nature發(fā)表的驚人的AFM圖像反映了紫色光合作用細(xì)菌Rhodobacter sphaeroides的光合作用復(fù)合體的組織情況,。專門的葉綠素蛋白的線性簇的一個(gè)網(wǎng)絡(luò),，被專門的、可利用光的蛋白連接在一起,，后者形成只有20-30納米寬的能量通道,。

The native architecture of a photosynthetic membrane

In photosynthesis, the harvesting of solar energy and its subsequent conversion into a stable charge separation are dependent upon an interconnected macromolecular network of membrane-associated chlorophyll–protein complexes. Although the detailed structure of each complex has been determined1-4, the size and organization of this network are unknown. Here we show the use of atomic force microscopy to directly reveal a native bacterial photosynthetic membrane. This first view of any multi-component membrane shows the relative positions and associations of the photosynthetic complexes and reveals crucial new features of the organization of the network: we found that the membrane is divided into specialized domains each with a different network organization and in which one type of complex predominates. Two types of organization were found for the peripheral light-harvesting LH2 complex. In the first, groups of 10–20 molecules of LH2 form light-capture domains that interconnect linear arrays of dimers of core reaction centre (RC)–light-harvesting 1 (RC–LH1–PufX) complexes; in the second they were found outside these arrays in larger clusters. The LH1 complex is ideally positioned to function as an energy collection hub, temporarily storing it before transfer to the RC where photochemistry occurs: the elegant economy of the photosynthetic membrane is demonstrated by the close packing of these linear arrays, which are often only separated by narrow 'energy conduits' of LH2 just two or three complexes wide.
 
 
Figure 1 AFM of native photosynthetic membranes. a, Large-scale view of several membrane fragments. b, Higher-magnification view showing a region of dimeric RC–LH1–PufX core complex arrays (red arrows) and associated LH2 complexes (green arrows). c, Three-dimensional view of core complex arrays surrounded by LH2 complexes. The inset at the bottom is a representation of the region denoted by the dashed box in the centre, using model structures derived from atomic resolution data2–4. A typical RC–LH1–PufX dimer is delineated in both images by a red outline and a representative LH2 complex by a green circle. Scale bar, 100 nm in all panels. For all images the z range is 6 nm (from darkest to lightest).

 
 
Figure 2 Membrane patches showing two types of arrangement of photosynthetic complexes. a, The circled region is composed mainly of LH2 complexes. b, Higher-magnification image of the same membrane patch in which an arrow points to an LH2 ring within the LH2-only domain. This higher-resolution scan clearly shows that there are no core complexes in these regions. Scale bar, 50 nm (a); 25 nm (b). 

 
 
Figure 3 Three-dimensional representation of a small region of membrane showing RC–LH1–PufX core complex dimers and monomers with associated LH2 complexes. Contact points for energy transfer between LH2 and RC–LH1–PufX complexes are indicated by green arrows. LH2 rings marked by asterisks are composed of nine units. The LH2 complex in the green circle is sandwiched between two RC–LH1–PufX complexes. The average tilt of seven LH1 complexes is 4.8°; the average height of LH1 above the lipid membrane is 1.4  0.3 nm (mean  s.d.). The maximum subunit height is 1.8  0.3 nm (n = 7) and the minimum subunit height is 1.1  0.1 nm (n = 7). The average tilt of three LH2 complexes is 3.8°. For LH2 the average height of LH2 rings above the lipid membrane is 1.5  0.2 nm (n = 11). For three tilted rings the maximum subunit height is 1.7  0.1 nm and the minimum subunit height is 1.2  0.1 nm. The average height of the reaction centre H subunit above the lipid membrane is 3.7  0.3 nm (n = 9). Scale bar, 10 nm.