Fig. 3: Structure of the GATOR2–CASTOR1 complex and its regulation by arginine.

a, Overview of the GATOR2–CASTOR1 co-complex. The top and bottom views along the C2 symmetry axis are presented. CASTOR1 binds to a dimeric interface formed by the MIOS brace region. Additional copies of CASTOR1 interact with the MIOS gloves via a single binding site but are probably not physiologically relevant. b, The GATOR2–CASTOR1 interface. Each protomer of CASTOR1 contacts one of the MIOS β-propellers that constitute the brace. c,d, Views of the interfaces between MIOS and CASTOR1. e, Validation of the GATOR2–CASTOR1 interface. Anti-Flag IPs were prepared from MIOS-deficient HEK293T cells transiently expressing the indicated cDNAs and analysed by immunoblotting for the indicated proteins. f, Validation of the GATOR2–CASTOR1 interface. Anti-HA IPs were prepared from HEK293T cells transiently expressing the indicated cDNAs and deprived of all amino acids for 60 min and analysed as in e. g–i, Arginine triggers a conformational change that results in the release of CASTOR1 from GATOR2. Comparison of the structures of arginine-bound CASTOR1 (light coral; 5I2C rebuilt) with arginine-free, GATOR2-bound CASTOR1 (salmon). g, The binding of arginine to CASTOR1 repositions helix α7 and reorients helix αL3. h, The binding of arginine to CASTOR1 reconfigures the loop enclosing the arginine-binding pocket and displaces helix α7 towards GATOR2 by one-half turn. i, Arginine binding, and translation of the CASTOR1 helix α7, reorients helix α3 and the ‘release loop’, which clashes with and displaces CASTOR1 from GATOR2. Data are representative of two independent experiments (e,f). For gel source data, see Supplementary Fig. 1.