High-resolution electron crystallography of bacterial transport proteins

次级转运蛋白将底物转运耦合到跨膜的离子梯度，形成一个非常大和多样化的蛋白质组。例如，基因组序列分析预测在大肠杆菌中存在大约160个二级转运体，其中大多数被预测有12个跨膜螺旋。尽管如此，几乎没有关于这些蛋白质的结构信息。我们已经获得了两种大肠杆菌转运蛋白NhaA和PutP的二维晶体，NhaA是一种Na+/H+逆向转运蛋白，PutP是一种Na+/Pro转运蛋白，最近利用电子结晶学计算了NhaA在8A分辨率下的投影图，为进一步的结构研究铺平了道路。由于大肠杆菌转运蛋白易于高效表达和纯化，因此为结构研究提供了极好的模型系统。获得这些蛋白质的结构信息将有助于深入了解其他次级转运蛋白(即，医学上重要的蛋白质，如5-羟色胺转运蛋白)的结构，并将对全面了解运输的分子机制至关重要，这在所有活着的生物体中都是至关重要的。

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.