4.A.3 The PTS Lactose-N,N'-Diacetylchitobiose-β-glucoside (Lac) Family
The Lac family includes several sequenced lactose (β-galactoside) porters of Gram-positive bacteria as well as the E. coli and Borrelia burgdorferi N,N'-diacetylchitobiose (Chb) porters. The former can transport aromatic β-glucosides and cellobiose as well as the chitin disaccharide, Chb. However, only Chb induces expression of the chb operon. While the Lac porters consist of two polypeptide chains (IIA and IICB), the Chb porters of E. coli and B. burgdorferi consist of three (IIA, IIB and IIC). In E. coli, the IIAChb protein has been shown to form a stable dimer both when phosphorylated and when unphosphorylated. The IIC domains of these permeases are believed to have a uniform topology with 8 TMSs (Nguyen et al., 2006).
In E. coli, the IIBChb is a monomer. Two IIBChb monomers associate with the IIAChb dimer. The structure of the IIB domain of the Chb porter has been determined both by NMR and by x-ray crystallography. It exhibits an α/β doubly wound superfold. This is different from the structure of the IIBGlc and IIBMan domains. IIBSgc, believed to function in pentose transport, is homologous to IIBLac and IIBChb. In B. subtilis, a PTS porter similar to the Chb porter of E. coli is believed to transport lichenan (a β-1,3;1,4 glucan) degradation products, oligosaccharides of 2-4 glucose units. The B. burgdorferi system is more similar to the Bacillus Lic system than the E. coli Chb system. The IIC domains of members of the Lac family are all more similar to each other than they are to those of the Glc, Bgl, Fru and Mtl families.