Association and dissociation of protein disulfide isomerase

Purified protein disulfide isomerase, homogeneous by SDS-PAGE, can be separated into two components by PAGE and by gel filtration. These two components, with the same amino-acid composition as well as N- and C-terminal sequences, are the tetramer and dimer of molecular weight 240 kDa and 120 kDa, respectively. The specific activity of the dimer is twice that of the tetramer. At 4 degrees C and pH 7.5 the purified dimer associates and the tetramer dissociates, both slowly and partially, to form a dimer-tetramer mixture. Treatment with dithiothreitol has only a minor effect on the dissociation of the tetramer indicating that the association is not through disulfide formation between the protomers. By prolonged treatment with 1% Triton X-100 or in strong salt solutions the tetramer dissociates to the dimer, but further dissociation to the monomer can only be effected in SDS or guanidine hydrochloride. These results suggest that apart from hydrogen bonds, hydrophobic forces and ionic interactions are mainly involved in the association of the protomers.