Projection structure and molecular architecture of OxIT, a bacterial membrane transporter

The major facilitator superfamily (MFS) represents the largest collection o f evolutionarily related members within the class of membrane 'carrier' pro teins. OAT, a representative example of the MFS, is an oxalate-transporting membrane protein in Oxalobacter formigenes. From an electron crystallograp hic analysis of two-dimensional crystals of OAT, we have determined the pro jection structure of this membrane transporter. The projection map at 6 Ang strom resolution indicates the presence of 12 transmembrane helices in each monomer of OxlT, with one set of six helices related to the other set by a n approximate internal two-fold axis. The projection map reveals the existe nce of a central cavity, which we propose to be part of the pathway of oxal ate transport. By combining information from the projection map with relate d biochemical data, we present probable models for the architectural arrang ement of transmembrane helices in this protein superfamily.