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 IIA^Man 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 (IIB^Man 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.

IIC^Man of E. coli has six established transmembrane α-helical spanners while IID^Man has only one with most of the polypeptide chain localized to the periplasm. These two proteins together are required for transport although IIC^Man is presumed to comprise all or most of the sugar transporting channel.