Structure of human Sp140 PHD finger: an atypical fold interacting with Pin1

Sp140 is a nuclear leukocyte-specific protein involved in primary biliary cirrhosis and a risk factor in chronic lymphocytic leukemia. The presence of several chromatin related modules such as plant homeodomain (PHD), bromodomain and SAND domain suggests a role in chromatin-mediated regulation of gene expression; however, its real function is still elusive. Herein we present the solution structure of Sp140-PHD finger and investigate its role as epigenetic reader in vitro. Sp140-PHD presents an atypical PHD finger fold which does not bind to histone H3 tails but is recognized by peptidylprolyl isomerase Pin1. Pin1 specifically binds to a phosphopeptide corresponding to the L3 loop of Sp140-PHD and catalyzes cis-trans isomerization of a pThr-Pro bond. Moreover co-immunoprecipitation experiments demonstrate FLAG-Sp140 interaction with endogenous Pin1 in vivo. Overall these data include Sp140 in the list of the increasing number of Pin1 binders and expand the regulatory potential of PHD fingers as versatile structural platforms for diversified interactions.