4.A.2 The PTS Fructose-Mannitol (Fru) Family
The Fru family is a large and complex family which includes several sequenced fructose, mannose and mannitol-specific porters as well as several putative PTS porters of unknown specificities. The fructose porters of this family phosphorylate fructose on the 1-position. Those of family 4.6 phosphorylate fructose on the 6-position. As is true of other members of the PTS-GFL superfamily, the IIC domains of these permeases probably have a uniform 8 TMS topology (Nguyen et al., 2006).
The IIA, IIB and IIC domains of the fructose- and mannitol-specific porters are demonstrably homologous. The IIB and IIC domains of the fructose porters are only distantly related to the corresponding domains of the mannitol porters. The IIB and IIC domains of these porters are homologous to those of the Glc family (TC #4.A.1) (Chang et al., 2004). However, the structure of the IIA domain of the mannitol porter of E. coli has been determined, and it proved to possess an α2β2α3 secondary structure, a structure which is very different from the β-sandwich structure of IIAGlc. Further, the IIC domains of the mannitol and fructose porters are as dissimilar from each other as they are from the glucose (TC #4.A.1) or lactose (TC #4.A.3) families.