Characterization of transmembrane segments 3, 4, and 5 of MalF by mutational analysis

MalF and MalG are the cytoplasmic membrane components of the binding protei n-dependent ATP binding cassette maltose transporter in Escherichia coli. T hey are thought to form the transport channel and are thus of critical impo rtance for the mechanism of transport. To study the contributions of indivi dual transmembrane segments of MalF, we isolated 27 point mutations in memb rane-spanning segments 3, 4, and 5. These data complement a previous study, which described the mutagenesis of membrane-spanning segments 6, 7, and 8. While most of the isolated mutations appear to cause assembly defects, L-3 23 Q in helix 5 could interfere more directly with substrate specificity. T he phenotypes and locations of the mutations are consistent with a previous ly postulated structural model of MalF.