Structural insight into the Rsa4–Nsa2 interaction. (A) Crystal structure of ctRsa4. Rsa4 consists of a UBL-like domain (light blue) followed by an eight-bladed β-propeller (left, b1–b8 in rainbow colors from dark blue to red; right, β-propeller rotated by 90° in dark blue) harboring the indicated α-helical insertion (purple) within blade 5. The highly conserved E117 (E114 in yeast) is also depicted, exposed on the surface of the UBL domain. (B) Crystal structure of the minimal scRsa4–scNsa2 complex. The surface view of scRsa4 is colored according to the charge calculated by PDB2PQR and APBS implemented in UCSF Chimera (left) with scNsa2 peptide (residues 86–95, red) bound into a hydrophobic pocket at the top site of the scRsa4 β-propeller (left). Also shown is a ribbon representation of the Nsa2 peptide (red) with its protruding residues bound to the scRsa4 β-propeller (blue) with hydrophobic (yellow) and charged residues involved in polar interactions (orange) at the rim of the propeller (right).