Monoclonal antibodies against the lac carrier protein from Escherichia coli. 1. Functional studies

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Abstract
The effects of various monoclonal antibodies against purified lac carrier protein on carrier-mediated lactose transport were studied in right-side-out membrane vesicles and in proteoliposomes reconstituted with purified lac carrier protein. Of > 60 monoclonal antibodies tested, only 1 antibody, 4B1, inhibits transport. The nature of the inhibition is highly specific in that the antibody inhibits only those transport reactions that involve net proton translocation (i.e., active transport, carrier-mediated influx and efflux under nonenergized conditions, and lactone-induced proton influx). The antibody has little effect on equilibrium exchange and no effect on generation of the proton electrochemical gradient or on the ability of the carrier to bind a high-affinity ligand. Therefore, the antibody alters the relationship between lactose and proton translocation at the level of the lac carrier protein. When entrance counterflow is studied with external [1-14C]lactose at saturating and subsaturating concentrations, it is apparent that antibody 4B1 mimics the effects of 2H2O; i.e., the antibody has no effect on the rate or extent of counterflow when external lactose is saturating but stimulates the efficiency of counterflow when external lactose is below the apparent Km. Therefore, the antibody either inhibits the rate of deprotonation or alters the equilibrium between protonated and deprotonated forms of the carrier. Monovalent Fab fragments prepared from antibody 4B1 inhibit transport in a manner that is similar qualitatively to that of the intact antibody. However, intact 4B1 is approximately twice as effective as the Fab fragments on a molar basis, suggesting that the intact molecule binds bivalently, while the Fab fragments bind 1:1.