Functional Interactions between the Subunits of the Lactose Transporter from Streptococcus thermophilus

Although the quaternary state has been assessed in detail for only a few members of the major facilitator superfamily (MFS), it is clear that multiple oligomeric states are represented within the MFS. One of its members, the lactose transporter LacS from Streptococcus thermophilus assumes a dimeric structure in the membrane and in vitro analysis showed functional interactions between both subunits when proton motive force (Δp)-driven transport was assayed. To study the interactions in further detail, a covalent dimer was constructed consisting of in tandem fused LacS subunits. These covalent dimers, composed of active and completely inactive subunits, were expressed in Escherichia coli, and initial rates of Δp-driven lactose uptake and lactose counterflow were determined. We now show that also in vivo, both subunits interact functionally; that is, partial complementation of the inactive subunit was observed for both transport modes. Thus, both subunits interact functionally in Δp-driven uptake and in counterflow transport. In addition, analysis of in tandem fused LacS subunits containing one regulatory LacS-IIA domain showed that regulation is primarily an intramolecular event.