The 5 A projection structure of the transmembrane domain of the mannitol transporter enzyme II

The uptake of mannitol in Escherichia coli is controlled by the phosphoenolpyruvate dependent phosphotransferase system. Enzyme II mannitol (EIIMtl) is part of the phosphotransferase system and consists of three covalently bound domains. IICMtl, the integral membrane domain of EIIMtl, is responsible for mannitol transport across the cytoplasmic membrane. In order to understand this molecular process, two-dimensional crystals of IICMtl were grown by reconstitution into lipid bilayers and their structure was investigated by cryo-electron crystallography. The IICMtl crystals obey p22121 symmetry and have a unit cell of 125 Ax65 A, gamma=90 degrees. A projection structure was determined at 5 A resolution using both electron images and electron diffractograms. The unit cell contains two IICMtl dimers with a size of about 40 Ax90 A, which are oriented up and down in the crystal. Each monomer exhibits six domains of high density which most likely correspond to transmembrane alpha-helices and cytoplasmic loops.