Structural Insight Into the SKP1-CUL1-FBXO3-RBX1 Complex

The cryo-EM structure of human SCF^FBXO3, which consists of CUL1, RBX1, SKP1 and FBXO3 was solved at a nominal resolution of 3.70 Å. Although a previous study reported the crystal structure of the FBXO3 ApaG domain, how FBXO3 is incorporated into the SCF complex remains elusive. In the cryo-EM structure of SCF^FBXO3, the F-box domain of FBXO3 primarily associates with SKP1 via extensive hydrophobic interactions and interacts with the N-terminal region of CUL1 via hydrophobic interactions. The weak cryo-EM map of the RBX1 globular region is close to the FBXO3 ApaG domain, suggesting that unmodified SCF^FBXO3 exhibits a closed conformation and that CUL1 neddylation is likely required to achieve high E3 activity. The structural study provides insight into the assembly of SCF^FBXO3 and its activation mediated by CUL1 neddylation.