Small-angle X-ray scattering studies on the photoreversible conformational change of phytochrome

光敏色素是绿色植物中一种重要的光感受器显色蛋白，对光具有多种形态发生和发育反应，包括调节光敏基因的表达。虽然最近确定了光敏色素的一级结构，但它的三级结构一直不清楚。由于高相对分子质量的光敏色素(约250 kDa)未能结晶，使得X射线结晶学在光敏色素研究中的应用变得困难。因此，小角X射线散射(SAXS)技术是研究光敏色素在溶液中结构的一种有用的技术。因此，我们利用SAXS开始了一系列光敏色素的结构研究。测定了分子的质量、回转半径、最大直径、平均电子密度和电子密度的涨落。利用所获得的结果，参照豌豆光敏色素在单体(n端59 kDa发色区域和C端55 kDa非发色区域)中分子结构域的结构，研究了豌豆光敏色素(单体相对分子质量为114 kDa)的红配基吸收形式的二聚体分子结构，并提出了“四叶形”模型。

Phytochrome is a noble photoreceptor chromoprotein in green plants for a variety of morphogenic and developmental responses to light including requlation of the expression of light-sensitive genes. Although the primary structure of phytochrome was recently determined, its tertiary structure has been obscure. It is difficult to apply X-ray crystallography to studies of phytochrome, since high molecular mass of phytochrome (about 250 kDa) has been failed to be crystallized. Hence, the small-angle X-ray scattering (SAXS) technique appears a useful technique for study on the structure of phytochrome in solution. We have, therefore, begun a series of structural studies of phytochrome using SAXS. we determined the molecular mass the radius of gyration, the maximum diameter, the average electron density, and the fluctuation of the elecron density in the molecule. Using the obtained results, the dimeric molecular structure of pea phytochrome (monomer molecular mass, 114 kDa) in the red-ligth-absorbing form was studied with reference to the structures of its molecular domains in a monomer (a n-terminal 59-kDa chromophoric domain and a C-terminal 55-kDa nonchromophoric domain) and a "four-leaved shape" model is proposed.