[Frontiers in Bioscience 17, 745-759, January 1, 2012]

The Na+/L-proline transporter PutP

Heinrich Jung1, Daniel Hilger1, Michael Raba1

1LMU Munich, Biocentre, Microbiology, Grosshaderner Strasse 2-4, Martinsried, D-82152, Germany


1. Abstract
2. Introduction
3. PutP - a member of the Na+/solute symporter family
4. Physiological significance of PutP
4.1. PutP in L-proline catabolism and osmoprotection
4.2. Role of PutP in bacteria host interactions
5. PutP and the LeuT-type structural fold
6. Structure-function relationships in PutP
6.1. The Na+ binding site
6.2. The L-proline binding site
7. Molecular mechanism of Na+/solute symport
7.1. Transport via alternating access
7.2. Impact of Na+ on the outward-facing cavity and substrate binding
7.3. The outer thin gate and substrate binding
7.4. Transition between outward- and inward-facing conformations
7.5. The inner thin gate and substrate release into the cytoplasm
8. Conclusions and perspectives
9. Acknowledgements
10. References


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 L-proline 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 bacteria-host 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 site-directed 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.