Characterization and partial purification of a rat brain protein which binds to GM1 ganglioside

Abstract

This study describes the characterization and partial purification of an endogenous protein from rat brain which binds with high affinity to GM1 ganglioside. The GM1 binding protein was identified by its ability to successfully compete with tritiated cholera toxin B subunit (CTB) for GM1 on liver membrane fragments. CTB was labelled to a high specific activity with ({dollar}\sp3{dollar}H) N-succinimidyl propionate while bound to a GM1 affinity column. Immobilization of the protein on this affinity column was done in order to avoid modifying any amino acids at the toxoid's GM1 binding site which might in turn reduce its binding affinity. The GM1 binding protein was isolated from 18-day-old whole rat brain homogenate fractionated by differential centrifugation; binding activity was identified in the 100,000g supernatant. Further fractionation by ammonium sulfate precipitation found nearly equal binding activity in the 25%-50% precipitate and 50% supernatant. DEAE ion exchange chromatography resulted in a single peak of binding activity well-separated from GM1 itself. The protein did not pass through an Amicon ultrafiltration membrane with a M{dollar}\sb{lcub}\rm r{rcub}{dollar} exclusion of 100,000 daltons. Binding protein activity was both trypsin sensitive and heat labile. Kinetic studies indicated that GM1 binds this protein with higher affinity than it binds cholera toxin. The only other tissues found to have such activity were intestines and spleen, which exhibited 0.2 and 0.1 times the level found in brain, respectively. This is the first description of an endogenous protein, with characteristics distinct from activator and transport proteins, demonstrating high binding affinity for GM1 ganglioside.