ATP-binding-cassette (ABC) subunit MalK of the binding protein-dependent transport system for maltose of Salmonella typhimurium and Escherichia coli is crucial to the transport process but also exhibits a repressing activity on other genes of the maltose regulon. The latter function has been attributed to a carboxy-terminal extension by which MalK differs in length from a prototype ABC protein. In order to define the boundaries of putative functional domains of MalK, we have analyzed pairs of N- and C-terminally truncated MalK proteins of S. typhimurium. Coexpressed half molecules of about equal lengths (MalKN1: residues 1 to 179; MalKC1: residues 179 to 369) restored the transport activity of a malK strain and displayed substantial regulatory activity. The same regulatory activity was obtained when malKC1 was expressed separately. These results indicate that a covalent linkage is not absolutely essential for function and that the protein might be composed of two structurally distinct entities. To elucidate further the minimal structural requirements for the regulatory function of MalK, we have studied chimeric proteins that have C-terminal portions of MalK fused to the corresponding amino-terminal fragments of its close homolog LacK. Functional analyses revealed that a fusion containing only the C-terminal extension of MalK (Q263 to V369) is sufficient to display half-maximal regulatory activity. This activity increased with the lengths of the MalK portions present in the chimeras. Furthermore, the failure of two chimeras to support maltose transport suggests a structurally critical region between residues 243 and 264. In the absence of a crystal structure, this work contributes to the understanding of the multiple functions of MalK.