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色蛋白，参与植物对光的多种形态发生和发育反应，包括调控光敏基因的表达。虽然光敏色素的一级结构最近才确定，但其三级结构尚不清楚。由于光敏色素的高分子量（约250 kDa）未能结晶，因此很难将x射线晶体学应用于光敏色素的研究。因此，小角x射线散射（SAXS）技术成为研究光敏色素溶液结构的一种有效技术。因此，我们利用SAXS开始了一系列光敏色素的结构研究。我们测定了分子质量、旋转半径、最大直径、平均电子密度和分子中电子密度的波动。利用所得结果，参照豌豆光敏色素在单体（n端59-kDa发色结构域和c端55-kDa非发色结构域）中的分子结构，研究了豌豆光敏色素（单体分子质量为114 kDa）在红光吸收形态下的二聚体分子结构，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.