Mutations within the nucleotide binding site of immunoglobulin-binding protein inhibit ATPase activity and interfere with release of immunoglobulin heavy chain

Abstract

Immunoglobulin-binding protein (BiP), a 70-kDa heat shock protein in the endoplasmic reticulum, binds transiently to nascent proteins, releasing them upon folding and assembly. The in vitro release of bound proteins from BiP requires ATP hydrolysis. Recently, the three-dimensional structure was solved for an ATP-hydrolyzing proteolytic 44-kDa fragment of a 71-kDa heat shock cognate protein, HSC71. Because of the high degree of homology in this region, BiP presumably forms a similar ATP binding structure. Amino-terminal deletions in BiP eliminated ATP-agarose binding. Alteration of a second potential ATP binding site had no effect, suggesting that only the HSC71-like site was capable of ATP binding. Crystallographic data from HSC71 implicated certain amino acids in interactions with the beta-phosphate, gamma-phosphate, and divalent cation of ATP. Mutation of each corresponding residue in BiP (Thr-37, Thr-229, and Glu-201) severely inhibited its ATPase activity. These BiP mutants were still capable of binding ATP and immunoglobulin heavy chains, suggesting that these mutations did not drastically alter the structure of BiP. They did however block the ATP-mediated release of heavy chains from BiP. Our results demonstrate that the structure of BiP in this region must be extremely similar to that elucidated for HSC71 and that mutations of residues proposed to interact with ATP block the ATP-mediated release of bound protein by inhibiting ATP hydrolysis.