The FP Domains of PI31 and Fbxo7 have the same Protein Fold but very Different Modes of Protein–Protein Interaction

Abstract

The FP (Fbxo7/PI31) domain was first identified on the basis of sequence similarity between the F-box protein (FBP) Fbxo7 and the proteasome inhibitor PI31. F-box only protein 7 (Fbxo7), a member of the Fbxo subfamily of FBPs, the best known function of Fbxo7 is the substrate-recognition subunit of the SCFFbxo7 E3 ubiquitin ligase that catalyzes the ubiquitination of hepatoma up-regulated protein (HURP) and inhibitor of apoptosis protein (IAP); PI31(Proteasome Inhibitor 31), a known regulatory subunit of the immunoproteasome which is an in vitro inhibitor of the 20S proteasome. Fbxo7 binds to PI31 with high affinity. The Fbxo7-PI31 interaction is mediated by heterodimerization of the FP domains. Recently, we have determined the crystal structure of the human Fbxo7 FP domain (181-335) at 2.0 Å and crystal structure of the longer PI31 FP domain (1-161) at 2.0 Å. The structure of PI31 FP domain is comparable to the Fbxo7 FP domain (including the C-terminal helix), indicating that the two FP domains share the same global fold. More significantly, structure of the Fbxo7 FP domain reveals a novel mode of inter-domain protein-protein interaction. The inter-domain interface of the Fbxo7 FP domain is best described as a αβ interface. While the inter-domain interface of PI31 FP domain can be described as α/β fold. Based on model building study, we propose that an interaction between the two FP domains of Fbxo7 and PI31 should be mediated by a αβ interface using the Fbxo7 FP domain α-helical surface and the PI31 FP domain β-sheet surface. Currently, we are trying to determine the Fbxo7-PI31 protein complex structure and study the interactions of Fbxo7 with its protein partners.