Secondary transporters couple substrate transport to ion gradients across membranes and form a very large and diverse group of proteins. For example, genome sequence analysis predicts the presence of ca. 160 secondary transporters in Escherichia coli, with the majority predicted to have 12 transmembrane helices. In spite of this, virtually no structural information is available for these proteins. Two-dimensional crystals of two E. coli transporters, NhaA, a Na+/H+ antiporter, and PutP, a Na+/proline symporter, have been obtained and recently allowed the calculation of a projection map of NhaA at 8 A resolution using electron crystallography, paving the way for further structural investigations. E. coli transporters provide excellent model systems for structural studies because of the ease of overexpression and purification. Obtaining structural information about these proteins will provide insight into the structure of other secondary transporters (i.e., medically important proteins such as the serotonin transporter) and will be essential to reach a complete understanding of the molecular mechanism of transport which is of fundamental importance in all living organisms.
