The 5åprojection structure of the transmembrane domain of the mannitol transporter enzyme II

Abstract

The uptake of mannitol in Escherichia coliis controlled by the phosphoenolpyruvate dependent phosphotransferase system. Enzyme II mannitol (EII Mtl) is part of the phosphotransferase system and consists of three covalently bound domains. IIC Mtl, the integral membrane domain of EII Mtl, is responsible for mannitol transport across the cytoplasmic membrane. In order to understand this molecular process, two-dimensional crystals of IIC Mtlwere grown by reconstitution into lipid bilayers and their structure was investigated by cryo-electron crystallography. The IIC Mtlcrystals obey p22 12 1symmetry and have a unit cell of 125Å×65Å, γ=90 °. A projection structure was determined at 5Åresolution using both electron images and electron diffractograms. The unit cell contains two IIC Mtldimers with a size of about 40Å×90Å, which are oriented up and down in the crystal. Each monomer exhibits six domains of high density which most likely correspond to transmembrane α-helices and cytoplasmic loops.