PI31 is a regulatory subunit of the immuno-proteasome which is an inhibitor of the 20 S proteasome in vitro.PI31 is also an F-box protein Fbxo7.Skp1 binding partner which requires an N terminal FP domain in both proteins for the interaction to occur via the FP beta sheets. The structure of PI31 FP domain contains a novel alpha/beta-fold and two intermolecular contact surfaces [1]. This is the N-terminal domain of the members.

1: Structure of a conserved dimerization domain within the F-box protein Fbxo7 and the PI31 proteasome inhibitor. Kirk R, Laman H, Knowles PP, Murray-Rust J, Lomonosov M, Meziane el K, McDonald NQ; J Biol Chem. 2008;283:22325-22335. PMID:18495667