INTRODUCTION
Flavonoids are a class of specialized plant metabolites that play pivotal roles in the survival of land plants in terrestrial environments. Currently, more than 6900 flavonoids with different structures have been identified and are known to have indispensable roles in land plants, not only owing to their antioxidant or UV-screening functions but also for their reproductions or defense mechanisms against various stresses.1 Isoflavonoids are one of the flavonoid groups produced by almost all legumes such asGlycine max (L.) Merr (soybean). Chalcone synthase (CHS), the first committed enzyme of the flavonoid biosynthetic pathway, catalyzes the consecutive condensations of three malonyl-CoA molecules into one p -coumaroyl-CoA molecule to produce 2’,4,4’,6’-tetrahydroxychalcone, which serves as a common flavonoid precursor (Fig. S1). We have previously reported that an isozyme of soybean CHS (termed GmCHS1) is a component of the soybean isoflavonoid metabolon and interacts with other components such as 2-hydroxyisoflavanone synthase isozymes, providing mechanistic evidence regarding the importance of the protein-protein interactions for the efficient production of 5-deoxyisoflavonoids in the soybean.2,3 More recently, we reported the interactions of CHS, including GmCHS1, with its nonenzymatic interacting partner, chalcone isomerase-like protein, in a wide variety of land plants.4 Interactions of this protein with CHS are widely conserved among land plants and reduce the intrinsic catalytic promiscuity of CHS to enhance the efficiency of flavonoid productionin planta .4 Although these observations indicate the functional significance of protein-protein interactions among CHS and other enzymes and proteins in the metabolons, there is no atomic-level structural information for protein-protein interactions that mediate flavonoid metabolon formation. Thus, in the first step of addressing this issue, we determined the crystal structure of recombinantly expressed GmCHS1 as one of the key components of a well-characterized flavonoid metabolon. We also determined the complex structure of GmCHS1 with bound naringenin, a product isomer of the CHS-catalyzed reaction, to better understand the catalytic mechanism of GmCHS1.