Structural Basis for Gibberellin Recognition by Its Receptor GID 1 Life Science : Structural Biology

Gibberellins are a large family of tetracyclic diterpenoid plant hormones that induce a wide range of plant growth responses including seed germination, stem elongation, leaf expansion, pollen maturation and induction of flowering. Gibberellins were first discovered by a Japanese plant pathologist, Eiichi Kurosawa, from the pathogenic fungus Gibberella fujikuroi in 1926 [1]. Kurosawa was working on rice plant diseases caused by this fungus, bakanae (foolish seedling), and found that some metabolite of this fungus might be responsible for the stimulated seedling growth. In 1935, an agricultural chemist, Teijiro Yabuta, isolated a crystalline active material that he named gibberellin [2]. In the 1950s, it was identified as natural components of noninfected plants and recognized as a plant hormone. Since then, some 136 different kinds of structurally similar gibberellins have been identified, although not all of them are biologically active as hormones in plants. Only a few gibberellins, such as GA1, GA3, and GA4 (Fig. 1), are bioactive hormones in plant [3]. In 2005, a research group led by Makoto Matsuoka discovered a nuclear receptor of gibberell ins, gibberellin-insensitive dwarf1 (GID1), from rice [4]. Unexpectedly, GID1 has sequence similarity to hormone-sensit ive l ipases (HSLs), which are enzymes involved in lipid metabolism. This fact raises the following two questions. (i) How different are their tertiary structures? (ii) How does GID1 manage to specif ically interact with bioactive gibberell ins while maintaining the conserved structure of the HSL family? We investigated the structural basis of gibberellin recognition in the rice, Oryza sativa GID1 (OsGID1) and revealed, on the basis of the structure, how GID1 has acquired the gibberellin reception ability that is lacking in HSLs [5]. The crystal structures of OsGID1 complexed with GA4 or GA3 have been solved by the Hg-SAD method and refined at 1.9 Å resolution. X-ray diffraction data for phasing and refinement were collected at beamline BL41XU. The structures revealed an α /β -hydrolase fold resembling that of HSLs (Fig. 2(a)). The gibberellin-binding cavity extends above Ser198 that corresponds to the catalytic residue of HSLs (Fig. 2(b)). Aside from having this catalytic Ser, the residues corresponding to the catalytic triad of HSLs, namely, Ser, His, Asp, are similarly arranged except for the replacement of His with Val in GID1 (Fig. 2(b)). The most notable difference between GID1 and HSL structures appears in the function of an amino-terminal lid (Fig. 2(a)). In HSLs, the lid covers the substrate binding site and opens upon substrate binding. In contrast, in GID1, the lid is open in the absence of the substrate