PTS_IIB_mannitol: subunit IIB of enzyme II (EII) of the mannitol-specific phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). In this system, EII is a mannitol-specific permease with two cytoplasmic domains (IIA and IIB) and a transmembrane channel IIC domain. The IIA, IIB, and IIC domains are expressed from the mtlA gene as a single protein, also known as the mannitol PTS permease, the mtl transporter, or MtlA. MtlA is only functional as a dimer with the dimer contacts occuring between the IIC domains. MtlA takes up exogenous mannitol releasing the phosphate ester into the cytoplasm in preparation for oxidation to fructose-6-phosphate by the NAD-dependent mannitol-P dehydrogenase (MtlD). 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 mannitol, chitobiose/lichenan, ascorbate, lactose, galactitol, fructose, and a sensory system with similarity to the bacterial bgl system.