Expression and purification of Shiga-like toxin II B subunits

Shiga-like toxins (SLTs), which are produced by certain strains of Escherichia coli, are composed of enzymatically active A and B subunit multimers responsible for the toxin's binding. We have previously purified large amounts of the SLT-I B subunit by using a hyperexpression vector in Vibrio cholerae under the control of the trc promoter. In this study we examined various expression vectors to maximize yields of the SLT-II B subunit. The SLT-II B subunit has been expressed by using both the T7 promoter and the tac promoter in E. coli. When expressed from a plasmid containing the structural gene for SLT-II B deleted of the leader sequence, SLT-II B was able to form multimers when cross-linked, although SLT-II B production from this plasmid was unreproducible. SLT-II B expressed in all three systems appeared to form unstable multimers, which did not readily bind to a monoclonal antibody which preferentially recognizes B subunit multimers. SLT-II B expression was not increased by moving any of the plasmids into V. cholerae. Polyclonal antibodies raised to SLT-II B in rabbits recognized B subunit in SLT-II holotoxin yet were poorly neutralizing. SLT-II B was also expressed as a fusion protein with maltose-binding protein and could be cleaved from maltose-binding protein with factor Xa. Although the expression vectors were able to make large amounts of SLT-II B, as determined by Western blotting (immunoblotting), the levels of purified SLT-II B subunit were low compared with those obtained previously for SLT-I B subunit, probably because of instability of the multimeric SLT-II B subunit.