Prokaryotic Solute/Sodium Symporters: Versatile Functions and Mechanisms of a Transporter Family

Abstract

The solute/sodium symporter family (SSS family; TC 2.A.21; SLC5) consists of integral membrane proteins that use an existing sodium gradient to drive the uphill transport of various solutes, such as sugars, amino acids, vitamins, or ions across the membrane. This large family has representatives in all three kingdoms of life. The human sodium/iodide symporter (NIS) and the sodium/glucose transporter (SGLT1) are involved in diseases such as iodide transport defect or glucose-galactose malabsorption. Moreover, the bacterial sodium/proline symporter PutP and the sodium/sialic acid symporter SiaT play important roles in bacteria-host interactions. This review focuses on the physiological significance and structural and functional features of prokaryotic members of the SSS family. Special emphasis will be given to the roles and properties of proteins containing an SSS family domain fused to domains typically found in bacterial sensor kinases.

Keywords: PutP; SLC5; bacterial sensor kinase; bacterial two-component systems; secondary transport; signal transduction; solute/sodium symport.

Figure 1

Possible roles of PutP in the proline metabolism of prokaryotes.

Figure 2

(A) Membrane topology of solute/sodium symporter (SSS) family transporters. SSS transporters are composed of 13 to 15 transmembrane domains (TMDs) connected by hydrophilic loops. The N terminus of the transporters is located on the outer site of the membrane. The transporters share the LeuT-type structural fold that is characterized by a core of ten TMDs that are arranged in five + five inverted repeats with an antiparallel orientation and a pseudosymmetry axis in the membrane plane [89]. The core domain starts at TMD 2 of SSS transporters (TMD 1 of SSS transporters is counted as TMD -1. (B) Ribbon representation of the three-dimensional structures of vSGLT, PmSiaT and EcPutP. The structures of vSGLT [84] and PmSiaT [64] are the result of X-ray analyses of respective crystals. The structure of EcPutP is based on homology modeling and experimental distance restraints obtained by site-directed spin labeling and EPR spectroscopy [90]. The figures were generated using UCSF Chimera. Chimera is developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco [91]. (A) was taken and modified from Jung et al., 2012 [31].

Figure 3

Sodium ion and substrate binding sites of vSGLT and PmSiaT. (A) Sideview of vSGLT and PmSiaT in the membrane plane. Core transmembrane domains (cTMDs) that coordinate with sodium ions and substrate are depicted in color, while the remaining TMDs are colored in grey. Substrates are shown as grey spheres colored by heteroatom and sodium ions are shown in purple. (B) Amino acids coordinating sodium ions or substrate. Amino acids are presented as grey sticks colored by heteroatom and substrates are presented as in (A). Amino acids positions highlighted in italics are considered to function as thin gates. Images were generated based on the crystal structures of vSGLT (3DH4) and PmSiaT (PDB 5NV9) using UCSF Chimera. Chimera is developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco [91]. Neu5Ac, N-acetylneuraminic acid.

Figure 4

Model of conformational states underlying the alternating access mechanism of SSS family transporters and other transporters with a LeuT-type structural fold. The conformational states observed in crystal structures of vSGLT, PmSiaT and AaLeuT are indicated. Comprehensive kinetic analyses particularly with SGLT [2] and to some extent with PutP [106] propose an ordered binding mechanism. Sodium ions bind first to the transporter in the outward-open apo state inducing a conformational alteration that facilitates substrate binding. The ion-substrate-protein complex undergoes further conformational alterations that lead, via an occluded state, to an opening of the ion and substrate binding sites towards the inside of the cell, and finally, to the release of both ligands into the cytosol. The resulting inwardly oriented apo state of the transporters changes to the outward-open conformation to allow for a new transport cycle. Reciprocal opening and closing of inwardly and outwardly oriented cavities may involve movement of thin gates (vSGLT: Y263; PmSiaT: I67, F78; AaLeuT: Y108, F253), rearrangements of interactions between TMDs as well as between TMDs and inner and outer loops (for example the loop connecting TMDs 7 and 8 [11,64,84]. out: outward-open conformation, occ: occluded conformation, in: inward-open conformation.

Figure 5

Prediction of organisms containing SSS family in association wih HisKa domain. SMART was used to identify proteins containing Pfam:ssf (SSS) and HisKa domains. After selection of representative organisms to be displayed for each group, a Newick formatted tree based on NCBI taxonomy was generated. The tree was visualized and modified using iTOL (Intreactive Tree Of Life) [116,117] tool and Adobe Illustrator.

Figure 6

Gram-positive organisms containing SSS family domain associated to HATPase_c. SMART database was used to identify proteins containing Pfam:ssf and HATPase_c (presented in the figure as “ATPa”) domains. Four isolates of Actinobacteria (5: Actinospica acidiphila, 6: Streptomyces griseorubens, 7: Streptomyces albus, 8: Streptomyces gilvosporeus) were selected and, as a comparison, three Pseudomonas spp. (2: Pseudomonas stutzeri, 3: Pseudomonas fluorescens Q2-87, 4: Pseudomonas corrugata) and one Archaea were also analyzed (1: Candidatus Nitrosocaldus cavascurensis). The ID of the isolates (SMART database) can be found in Table S2.

Figure 7

Domain structure of CbrA and MxtR as typical SSS-dependent sensor kinases with the according response regulators. The N-terminal portion of the sensor kinases forms 13 TMDs that resemble the topology of an SSS family member, while the C-terminal portions contains domains typically found in sensor kinases. TM, also called TMD, transmembrane domain; STAC, SLC and TCST-Associated Component; PAS, Per-Arnt-Sim; DHp, histidine phosphotransfer domain, also referred to as HisKA; CA, catalytic ATP-binding domain, also referred to as HATpase_c; REC, receiver; DBD, DNA binding domain (for “sigma-54 interaction domain” predicted by ScanProsite; HTH, helix-turn-helix. Images created with DOG 2.0 illustrator [140]. Domain prediction relies on SMART [113,114] and [133].