InterPro: IPR013012 Phosphotransferase system, EIIB component, type 3

The phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) [1, 2] is a major carbohydrate transport system in bacteria. The PTS catalyses the phosphorylation of incoming sugar substrates and coupled with translocation across the cell membrane, makes the PTS a link between the uptake and metabolism of sugars.

The general mechanism of the PTS is the following: a phosphoryl group from phosphoenolpyruvate (PEP) is transferred via a signal transduction pathway, to enzyme I (EI) which in turn transfers it to a phosphoryl carrier, the histidine protein (HPr). Phospho-HPr then transfers the phosphoryl group to a sugar-specific permease, a membrane-bound complex known as enzyme 2 (EII), which transports the sugar to the cell. EII consists of at least three structurally distinct domains IIA, IIB and IIC [3]. These can either be fused together in a single polypeptide chain or exist as two or three interactive chains, formerly called enzymes II (EII) and III (EIII).

The first domain (IIA or EIIA) carries the first permease-specific phosphorylation site, a histidine which is phosphorylated by phospho-HPr. The second domain (IIB or EIIB) is phosphorylated by phospho-IIA on a cysteinyl or histidyl residue, depending on the sugar transported. Finally, the phosphoryl group is transferred from the IIB domain to the sugar substrate concomitantly with the sugar uptake processed by the IIC domain. This third domain (IIC or EIIC) forms the translocation channel and the specific substrate-binding site.

An additional transmembrane domain IID, homologous to IIC, can be found in some PTSs, e.g. for mannose [1, 3, 4, 5].

According to structural and sequence analyses, the PTS EIIB domain (EC:2.7.1.69) can be divided in five groups [6, 7, 8]:

• The PTS EIIB type 1 domain, which is found in the Glucose class of PTS, has an average length of about 80 amino acids. It forms a split alpha/beta sandwich composed of an antiparallel sheet (beta 1 to beta 4) and three alpha helices superimposed onto one side of the sheet. The phosphorylation site (Cys) is located at the end of the first beta strand on a protrusion formed by the edge of beta 1 and the reverse turn between beta 1 and beta 2 [6].
• The PTS EIIB type 2 domain, which is found in the Mannitol class of PTS, has an average length of about 100 amino acids. It consists of a four stranded parallel beta sheet flanked by two alpha helices (alpha 1 and 3) on one face and helix alpha 2 on the opposite face, with a characteristic Rossmann fold comprising two right-handed beta-alpha-beta motifs. The phosphorylation site (Cys) is located at the N terminus of the domain, in the first beta strand.
• The PTS EIIB type 3 domain, which is found in the Lactose class of PTS, has an average length of about 100 amino acids. It is composed of a central four-stranded parallel open twisted beta sheet, which is flanked by three alpha helices on the concave side and two on the convex side of the beta sheet. The phosphorylation site (Cys) is located in the C-terminal end of the first beta strand [7].
• The PTS EIIB type 4 domain, which is found in the Mannose class of PTS, has an average length of about 160 amino acids. It has a central core of seven parallel beta strands surrounded by a total of six alpha-helices. Three helices cover the front face, one the back face with the remaining two capping the central beta sheet at the top and bottom. The phosphorylation site (His) is located at the suface exposed loop between strand 1 and helix 1 [7].
• The PTS EIIB type 5 domain, which is found in the Sorbitol class of PTS, has an average length of about 190 amino acids. The phosphorylation site (Cys) is located in the N terminus of the domain.

The EIIB type 3 domain is often found downstream of the EIIC type 3 domain.