4.A.5 The PTS Galactitol (Gat) Family
The only well-characterized member of the Gat famly is the galactitol permease of E. coli (Noblemann and Lengeler, 1995). However, a homologous IIC protein (38% identity) from Listeria monocytogenes has been shown to be required for D-arabitol fermentation (Saklani-Jusforgues et al., 2001). It functions together with IIAGat and IIBGat homologues. IICGat is distantly related to IICSgc of E. coli (Reizer et al., 1996); IIAGat is distantly related to IIASga and IIASgc of E. coli as well as IIAMtl and IIAFru. IIBGat is distantly related to IIBSga and IIBSgc of E. coli. Domains in the LicR/CelR family of transcriptional activators show C-terminal domains exhibiting weak sequence similiarity to IIBGat and IIAGat (Tchieu et al., 2001). The biochemistry of this family is poorly defined. Howeever, the 3-d structure of GatB has been solved. GatB consists of a central four-stranded parallel beta-sheet flanked by alpha-helices on both sides. The active site cysteine of GatB is located at the beginning of an unstructured loop between beta1 and alpha1 that folds into a P-loop-like structure. This structural arrangement shows similarities with other IIB subunits as noted above, but also with mammalian low molecular weight protein tyrosine phosphatases (LMW PTPase) and arsenate reductase (ArsC) (Volpon et al. 2006).The Gat Family is related to the L-Asc Family (TC# 4.A.7) (Hvorup et al. 2003; Saier et al. 2005).