PTS_IIB: subunit IIB of enzyme II (EII) is the central energy-coupling domain of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). In the multienzyme PTS complex, EII is a carbohydrate-specific permease consisting of two cytoplasmic domains (IIA and IIB) and a transmembrane channel IIC domain. The IIB domain fold includes a central four-stranded parallel open twisted beta-sheet flanked by alpha-helices on both sides. The seven major PTS systems with this IIB fold include chitobiose/lichenan, ascorbate, lactose, galactitol, mannitol, fructose, and a sensory system with similarity to the bacterial bgl system. The PTS is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, polyols, amino sugars, and other sugar derivatives. The four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB), can phosphorylate or interact with numerous non-PTS proteins thereby regulating their activity.