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 10 TMSs (Cao et al. 2011; Vastermark and Saier 2016).

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.

This family belongs to the PTS-GFL Superfamily.



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TC#NameOrganismal TypeExample
4.A.3.1.1Lactose (Lac) porter Bacteria Lactose (Lac) IICB-IIA complex of Staphylococcus aureus
IIA (LacF) (P02909)
IIBC (LacE) (P11162)

Lactose PTS group translocator #1, IIA/IICB (Francl et al. 2012).


Lactose PTS porter #1of Lactobacillus gasseri


Lactose PTS group translocator #2, IIA/IICB (Francl et al. 2012).


Lactose PTS porter #2 of Lactobacillus gasseri


TC#NameOrganismal TypeExample
4.A.3.2.1N,N'-diacetylchitobiose (Chb) porter (also transports cellobiose; Kachroo et al., 2007).Bacteria N,N'-diacetylchitobiose (Chb) IIC-IIB-IIA complex of E. coli
IIA (ChbA), spP17335
IIB (ChbB), spP17409
IIC (ChbC), spP17334

Oligo β-(gluco)mannoside (derived from (gluco)mannan))-specific PTS transporter, GmuABC (YdhNMO). The glucomannan utilization operon (gmuBACDREFG, formerly ydhMNOPQRST) of Bacillus subtilis has been characterized (Sadaie et al. 2008). Transcription of the operon is induced by konjac glucomannan and requires the last mannanase gene (gmuG). Cellobiose and mannobiose, possible degradation products of glucomannan by GmuG, are strong inducers of transcription. An internal regulatory gene (gmuR) encodes a repressor of the operon, as disruption of this gene enhances transcription of the operon in the absence of inducers. The expression of the glucomannan utilizing operon is thus induced by degraded glucomannan products, and repressed by an internal repressor (Sadaie et al. 2008).

GmuABC of Bacillus subtilis
GmuA, IIA, 110 aas, O05506
GmuB, IIB, 103 aas, O05505
GmuC, IIC, 422 aas and 10 TMSs, O05507

4.A.3.2.2Lichenan oligosaccharide (Lic) porter Bacteria Lichenan oligosaccharide (Lic) IIC-IIB-IIA complex of Bacillus subtilis
IIA (LicA), spP46319
IIB (LicB), spP46318
IIC (LicC), spP46317

N,N'-diacetylchitobiose porter, ChbABC


N,N'diacetylchitobiose (Chb) IIC-IIB-IIA complex of Borrelia burgdorferi
IIA (ChbA), AAC66323
IIB (ChbB), AAC66322
IIC (ChbC), AAC66324


The cellobiose-specific (PtcA-PtcB-CelB) porter (Kowalczyk et al., 2008 ).


PtcA-PtcB-CelB of Lactococcus lactis:
CelB (C)(Q9CJ32) PtcB (B)(Q9CIF0) PtcA (A)(Q9CIE9)


The N,N' -diacetylchitobiose Enzyme II (Toratani et al., 2008) (>80% identical to the E. coli enzyme (4.A.3.2.1)).  The IIA protein is the Serratia glucose IIA which is nearly identical to the E. coli IIAGlc (Crr).


The N,N' -diacetylchitobiose Enzyme II of Serratia marcescens
IIA (ChbA) - Q8L3C4
IIB (ChbB) - Q8L3C3
IIC (ChbC) - Q8L3C2

4.A.3.2.6Glucosamine β(1→4) glucosamine (GlcN)2 (glucosamine dimer) transporter, VC1281-1283 (IIB; IIC; IIA, respectively) (Meibom et al., 2004)Proteobacteria(GlcN)2 transporter of Vibrio cholerae (IIA-IIB-IIC)

N,N'-diacetylchitobiose (cellobiose) PTS permease, CelABC (IIABC).  Essential for normal virulence and biofilm formation by K. pneumoniae which causes pyogenic liver abscesses (Wu et al. 2012).  88% identical to the diacetylchitobiose II (4.A.3.2.1) of E. coli.  These two proteins are probably orthologous. 


CelABC of Klebsiella pneumoniae


N-, N'-diacetylchitobiose PTS permease, IIABCChb (Also called IIABCCel for cellbiose). The crystal structure of the 10 TMS IIC membrane component has been determined by x-ray crystallography (Cao et al. 2011).


ChbIIABC of Bacillus cereus
ChbA (CelA); IIB (100 aas) (Q72XP9)
ChbB (CelB); IIC (433 aas) (Q72XQ0)
ChbC (CelC); IIA (106 aas) (Q72XQ1)


PTS-type lactose transporter, IIC-IIB-IIA (Imai and Hall 1981; Hall et al. 1982). 


Lactose permease of Klebsiella pneumoniae