Crystal structures of the regulatory subunit of Thr-sensitive aspartate kinase from

高熱菌由来のスレオニン感受性アスパラギン酸リン酸化酵素の制御ドメインの結晶構造

吉田 彩子*; 富田 武郎*; 河野 秀俊; 伏信 進矢*; 葛山 智久*; 西山 真*

Yoshida, Ayako*; Tomita, Takeo*; Kono, Hidetoshi; Fushinobu, Shinya*; Kuzuyama, Tomohisa*; Nishiyama, Makoto*

Crystal structures of the regulatory subunit (TtAK) of Thr-sensitive aspartate kinase (AK; EC 2.7.2.4) from Thermus thermophilus were determined at 2.15 ; in the Thr-bound form(TtAK-Thr) and at 2.98 ; in the Thr-free form (TtAK-free). Although both forms are crystallized as dimers, the contact surface area of the dimer interface in TtAK-free (3,200 ) is smaller than that of TtAK-Thr (3,890 ). Sedimentation equilibrium analyzed by ultracentrifuge revealed that TtAK is present in equilibrium between a monomer and dimmer and Thr binding shifts the equilibrium to dimer formation. In the absence of Thr, outward shift of b strands near the Thr-binding site (site 1) and concomitant loss of the electron density of the loop region between 3 and 4 near the Thr-binding site are observed. Based on the crystal structures, the regulatory mechanism by Thr is discussed. TtAK has higher thermostability than the regulatory subunit of AK (CgAK) with a difference in denaturation temperature () of 40C. Comparison of the crystal structures of TtAK and CgAK showed that the well-packed hydrophobic core and highproline content in loops contribute to the high thermostability of TtAK.