Association of the βγ Subunits of Trimeric GTP-Binding Proteins with 90-kDa Heat Shock Protein, hsp901

Abstract

GTP-binding proteins (G proteins), predominantly located at the inner surface of the plasma membranes of mammalian cells, dissociate into their constituent alpha and beta gamma subunits upon stimulation of G protein-coupled receptors by agonists. In the present studies, cytoplasmic proteins which might have an affinity for the dissociated beta gamma subunits were investigated by means of beta gamma subunit-immobilized affinity-column (beta gamma-immobilized column) chromatography. When soluble fractions obtained from various materials including rat liver, bovine brain, and HL-60 cells were applied to a beta gamma-immobilized column, some proteins were specifically eluted from the column with high-salt and detergent-containing solutions. One of the beta gamma subunit-binding proteins, of which the molecular weight was approximately 93,000 on SDS-PAGE, appeared to be commonly present in all tissues tested. The 93-kDa beta gamma-binding protein was identified as 90-kDa heat shock protein, hsp90, based on the findings of its partial amino acid sequences and its immunoreactivity to a monoclonal anti-hsp90 antibody. The brain hsp90 inhibited beta gamma-supported pertussis toxin-catalyzed ADP-ribosylation of alpha subunits. The hsp90 was also capable of binding to beta gamma subunits which had been reconstituted into phospholipid vesicles. The binding of hsp90 to beta gamma subunits was inhibited by the addition of GDP-bound alpha subunits, but not by GTP gamma S-bound ones. These results suggested that hsp90 could associate functionally with free beta gamma subunits dissociated from trimeric G proteins in vitro.