MalK forms a dimer independent of its assembly into the MalFGK2 ATP-binding cassette transporter of Escherichia coli

The maltose transport complex (MTC) is a member of the ATP-binding cassette superfamily of membrane transport proteins and is a model for understanding the folding and assembly of hetero oligomeric membrane protein complexes. The MTC is made up of two integral membrane proteins, MalF and MalG, and a peripheral membrane protein, MalK, These proteins associate with a stoichiometry of 1:1:2 to form the complex MalFGK(2). In our studies of the oligomerization of this complex, we have shown that the ATP-binding component, MalK, forms a dimer in the absence of MalF and MalG, Epitope-tagged MalK coimmunoprecipitated with wild-type MalK, indicating that the MalK protein forms an oligomer, The relative amounts of tagged and wild-type MalK that were present in the whole cell extracts and in the immunoprecipitated complexes show that the MalK oligomer is a dimer, These hetero-oligomers can also be formed in vitro by mixing two extracts, each containing either tagged or wild-type MalK. The dimerization of MalK was also demonstrated in vivo using the bacteriophage lambda repressor fusion assay. The formation of a MalK dimer in the absence of MalF and MalG may represent an initial step in the assembly pathway of the MTC.