The ATP-binding-cassette (ABC) protein LacK of Agro-bacterium radiobacter displays high sequence similarity to the MalK subunit of the Salmonella typhimurium maltose-transport system (MalFGK2). We have used LacK as a tool to identify sites of interaction of MalK with the membrane-integral components MalF and MalG. Small amounts of LacK, resulting from the expression of the plasmid-borne lacK gene, proved to be sufficient for partial restoration of growth of a malK strain of S. typhimurium on maltose. LacK failed to substitute for MalK in regulating the expression of maltose-inducible genes but the hybrid complex MalFGLacK2 was sensitive to inducer exclusion. The lacK gene also complemented a ugpC mutant of Escherichia coli to growth on sn-glycerol-3-phosphate as the phosphate source. Partially purified LacK exhibited a spontaneous ATPase activity comparable to that of MalK. A MalK′–′LacK chimeric protein was isolated (by in vivo recombination) in which the N-terminal 140 amino acids of MalK are fused to residues 141–363 of LacK. The protein substituted for MalK in maltose transport considerably better than LacK. Furthermore, random mutagenesis of the plasmid-borne lacK gene yielded three clones that were superior to wild-type lacK in complementing a malK mutation. Single mutations (V114M or L123F) substantially improved the growth of a malK strain on maltose, whereas a double mutation (L123F, S295N) resulted in growth and transport rates that were indistinguishable from those obtained with MalK. In contrast, the introduction of the single change S295N into LacK had no effect but combination with the V114M mutation led to a further twofold increase in transport activity. These results indicate that a putative helical domain in MalK, encompassing residues 89–140, is crucial for a functional, high-affinity interaction with MalF and MalG.