F-box proteins are the substrate-recognition components of the Skp1-Cul1-F box protein (SCF) E3 ubiquitin ligases. Here we report a structural relationship between Fbxo7, a component of the SCF(Fbxo7) E3 ligase, and the proteasome inhibitor PI31. SCF(Fbxo7) is known to catalyze the ubiquitination of hepatoma-up-regulated protein (HURP) and the inhibitor of apoptosis (IAP) protein but also functions as an activator of cyclin D-Cdk6 complexes. We identify PI31 as an Fbxo7.Skp1 binding partner and show that this interaction requires an N-terminal domain present in both proteins that we term the FP (Fbxo7/PI31) domain. The crystal structure of the PI31 FP domain reveals a novel alpha/beta-fold. Biophysical and mutational analyses are used to map regions of the PI31 FP domain mediating homodimerization and required for heterodimerization with Fbxo7.Skp1. Equivalent mutations in Fbxo7 ablate interaction with PI31 and also block Fbxo7 homodimerization. Knockdown of Fbxo7 does not affect PI31 levels arguing against PI31 being a substrate for SCF(Fbxo7). We present a model for FP domain-mediated dimerization of SCF(Fbxo7) and PI31.