Studies on the Mechanism of Heat Aggregation of Human γG

Abstract

The intact proteins and Fab and Fc fragments of normal γG and myeloma proteins of the four γG subclasses were studied for their susceptibility to heat aggregation. It was found that in the case of normal γG and myeloma proteins of the two major subclasses, γG1 and γG2, aggregation of the intact protein correlated with aggregation of the Fab fragment, whereas the Fc fragments of these proteins did not aggregate appreciably. In the case of the two minor subclasses, γG3 and γG4, the Fc fragment was also a contributing factor in the formation of heat aggregates. Aggregation could be partially reversed (54% to 73%) by treatment with 8 M urea, indicating that both disulfide bonds and non-covalent interactions were responsible for the formation of the aggregates. It was also found that heat aggregation of γG1 protein leads to a conformational change in the Fab which renders it extremely susceptible to proteolysis. The significance of these findings with respect to the mechanism by which aggregation results in the enhanced biologic activity of the Fc fragment is discussed.