The Na+/L-proline transporter PutP

Abstract

The Na+/L-proline transporter PutP is a member of the Na +/solute symporter family (TC 2A.21, SLC5), which contains several hundred proteins of pro- and eukaryotic origin. Within the family, the capability of Lproline uptake is restricted to proteins of prokaryotes. PutP contributes to the use of L-proline as a nutrient. In addition, the transporter may supply cells with compatible solute during adaptation to osmotic stress. Based on these and other functions, PutP is of significance for various bacteriahost interactions including the virulence of human pathogens. A homology model of Escherichia coli PutP was generated based on the crystal structure of the Vibrio parahaemolyticus Na+/galactose symporter. According to the model, PutP has a core structure of five plus five transmembrane domains forming an inverted repeat similar as originally revealed by the crystal structure of the Na+/leucine transporter LeuT. The homology model is experimentally verified by Cys cross-linking and sitedirected spin labeling in combination with electron paramagnetic resonance spectroscopy. The putative sites of Na+ and L-proline binding are described, and a putative transport mechanism is discussed.