BIP co-chaperone MTJ1/ERDJ1 interacts with inter-α-trypsin inhibitor heavy chain 4

Abstract

MTJ1/ERdj1 and its human homologue HTJ1 are membrane proteins that interact with the molecular chaperone BiP through their J-domain. HTJ1 also contains a C-terminal cytosolic region of unknown function that consists of two SANT domains separated by a spacer region. We recently showed that the second SANT domain of HTJ1 (SANT2) binds to α1-antichymotrypsin and alters its serpin activity [B. Kroczynska, C.M. Evangelista, S.S. Samant, E.C. Elguindi, S.Y. Blond, The SANT2 domain of the murine tumor cell DnaJ-like protein 1 human homologue interacts with α1-antichymotrypsin and kinetically interferes with its serpin inhibitory activity, J. Biol. Chem. 279 (2004) 11432–11443]. Here, we identified a new variant of human inter-α-trypsin inhibitor heavy chain 4 (ITIH4) that also interacts with the SANT2 domain of HTJ1. Biochemical, mutagenesis, and fluorescence studies demonstrate that SANT2 binds to a carboxyl-terminal fragment that corresponds to the last third of the new ITIH4 isoform sequence (residues 588–930). ITIH4 and MTJ1 co-immunoprecipitate from total liver protein extracts and SANT2 protects the ITIH4 588–930 recombinant fragment from being processed by kallikrein in vitro. This work reveals that the SANT2 domain of HTJ1 is a genuine protein–protein interaction module.