Monoclonal antibodies reveal lamB antigenic determinants on both faces of the Escherichia coli outer membrane.

LamB protein is involved in the transport of maltose across the outer membrane and constitutes the receptor for phage lambda. In this study we characterized six previously described anti-LamB monoclonal antibodies (mAbs). Four of these, the E-mAbs, recognized determinants that were exposed at the cell surface, whereas the other two, the I-mAbs, recognized determinants which were not exposed. Competition experiments demonstrated that the domains recognized by these two classes of mAbs were completely distinct. In addition, the E-mAbs prevented LamB from neutralizing phage lambda in vitro and protected LamB against proteolytic degradation, whereas the I-mAbs had no such effects. The E-mAbs have been shown previously to constitute two classes: some E-mAbs inhibit maltose transport in vivo, and others do not. Immunoelectron microscopy demonstrated that the I-mAbs also define at least two types of determinants. One of these, which is accessible in membrane fragments from a mutant (lpp) devoid of lipoprotein but not in membrane fragments from an lpp+ strain, probably corresponds to a region of LamB that is involved in the interactions with peptidoglycan. The other determinant, which is fully accessible in LamB-peptidoglycan complexes and in LamB-containing phospholipid vesicles but only slightly accessible in membrane fragments from an lpp mutant, is probably located very close to the inner surface of the outer membrane. LamB also contains at least one additional determinant, which (i) is exposed at the inner surface of the membrane, (ii) is accessible to antibodies in membrane fragments from an lpp+ strain, and (iii) may be involved in the interaction of LamB with the periplasmic maltose-binding protein.