4.A.6 The PTS Mannose-Fructose-Sorbose (Man) Family
The Man (PTS splinter group) family is unique in several respects among PTS porter families. (1) It is the only PTS family in which members possess a IID protein; (2) It is the only PTS family in which the IIB constituent is phosphorylated on a histidyl rather than a cysteyl residue. (3) Its porter members usually exhibit broad specificity for a range of sugars, rather than being specific for just one or a few sugars. The mannose porter of E. coli, for example, can transport and phosphorylate glucose, mannose, fructose, glucosamine, N-acetylglucosamine, and N-acteylmannosamine (Plumbridge and Vimr, 1999).
The structure of the E. coli IIAMan domain has been shown to exhibit an α/β doubly wound superfold. The IIB domain also exhibits an α/β doubly wound superfold, but it is very dissimilar from that of the IIA domain. Instead, it has the same topology as phosphoglyceromutase. Since both proteins (IIBMan and PGM) catalyze phosphoryl transfer with a phosphohistidine intermediate, both proteins show a similar distribution of active site residues, and both exhibit similar structures, they are probably homologous.
IICMan of E. coli has six established transmembrane α-helical spanners while IIDMan has only one with most of the polypeptide chain localized to the periplasm. These two proteins together are required for transport although IICMan is presumed to comprise all or most of the sugar transporting channel.