Contributions of Coulombic and Hofmeister Effects to the Osmotic Activation of Escherichia coli Transporter ProP

Abstract

Osmosensing transporters mediate osmolyte accumulation to forestall cellular dehydration as the extracellular osmolality increases. ProP is a bacterial osmolyte-H+ symporter, a major facilitator superfamily member, and a paradigm for osmosensing. ProP activity is a sigmoid function of the osmolality. It is determined by the osmolality, not the magnitude or direction of the osmotic shift, in cells and salt-loaded proteoliposomes. The activation threshold varies directly with the proportion of anionic phospholipid in cells and proteoliposomes. The osmosensory mechanism was probed by varying the salt composition and concentration outside and inside proteoliposomes. Data analysis was based on the hypothesis that the fraction of maximal transporter activity at a particular luminal salt concentration reflects the proportion of ProP molecules in an active conformation. ProP attained the same activity at the same osmolality when diverse, membrane-impermeant salts were added to the external medium. Contributions of Coulombic and/or Hofmeister salt effects to ProP activation were examined by varying the luminal salt cation (K+ and Na+) and anion (chloride, phosphate, and sulfate) composition and then systematically increasing the luminal salt concentration by increasing the external osmolality. ProP activity increased with the sixth power of the univalent cation concentration, independent of the type of anion. This indicates that salt activation of ProP is a Coulombic, cation effect resulting from salt cation accumulation and not site-specific cation binding. Possible origins of this Coulombic effect include folding or assembly of anionic cytoplasmic ProP domains, an increase in local membrane surface charge density, and/or the juxtaposition of anionic protein and membrane surfaces during activation.

Do soft anions promote protein denaturation through binding interactions? A case study using ribonuclease A

“…Coulombic salt effects on protein stability are oen predicted to scale with the Debye-Huckel limiting law, i.e., linearly depend on the square root of the solvent ionic strength. [72][73][74][75][76] It has however been shown that there is no real justication for this assumption 72 and a more thorough analysis of salt-induced coulombic effects on protein folding free energy predicts that for the monovalent salts in this study they following functional form: 9,40 Salt-induced Coulomb effects ¼ DDG unfolding…”

“…Most systematic analyses of salt effects on proteins postulate that salt addition inuences the stability of proteins through nonspecic Coulomb and ion-specic Hofmeister effects. 9,40,62 The formalism of Record et al, 9,33,[36][37][38][39][40]62,63 quanties Hofmeister effects on the protein folding process by "m-values", the derivative of the free energy of unfolding with respect to salt concentration: ‡…”

“…This notwithstanding, it has been shown that coarsely decomposing the protein surface into three distinct chemical types (aliphatic hydrocarbon, aromatic hydrocarbon and polar amide) and analyzing the salt-induced changes in protein folding in terms of ion interactions with these three surface types, can provide a useful semi-quantitative interpretation of the observed ion specicities in protein folding. 9,33,40 Measurements of protein stability have shown that the so anions thiocyanate, iodide and bromide signicantly destabilize the folded state of protein. 9,[41][42][43][44][45] At a rst glance, it may seem logical to interpret the effects of these so ions in terms of their interaction with aliphatic hydrocarbon, aromatic hydrocarbon and polar amide surfaces; however, a series of elegant studies on model polypeptides that combined thermodynamic, spectroscopic and theoretical approaches call this interpretation into question.…”

EctD-mediated biotransformation of the chemical chaperone ectoine into hydroxyectoine and its mechanosensitive channel-independent excretion

“…Escherichia coli cannot synthesize ectoine but it can import it via the osmotically inducible ProP and ProU osmostress protectant uptake systems [ 60 , 61 ]. ProP is a proton/solute symporter and a member of the major facilitator (MFS) superfamily [ 62 ], whereas ProU belongs to the multi-component ABC-type of transport systems [ 63 , 64 ]. Ectoine is likely to permeate across the E. coli outer membrane by diffusion through the general porins OmpC and OmpF (Fig.…”

OpuF, a New Bacillus Compatible Solute ABC Transporter with a Substrate-Binding Protein Fused to the Transmembrane Domain

“…For energetic reasons (18), the import of preformed compatible solutes is preferred over their synthesis, and consequently, uptake systems for these stress-relieving compounds are frequently a cornerstone of the cellular osmostress response systems of bacteria (1, 4-6, 19, 20). A considerable number of compatible solute transporters have been identified in bacteria that belong to different transporter families, as follows: binding-protein-dependent ATP-binding cassette (ABC) systems (21) (e.g., the ProU, OpuA, OpuB, OpuC, BusA, OsmU, OusA, and Prb transporters [22][23][24][25][26][27][28][29][30]), members of the major facilitator superfamily (MFS) (31) (e.g., the ProP and OusA transporters [32][33][34]), members of the betaine-choline-carnitine transport (BCCT) systems (35) (e.g., the OpuD, BetP, BetS, EctT, EctP, EctM, and BetM transporters [33,[36][37][38][39]), members of the sodium-solute-symporter (SSS) family (40) (e.g., OpuE [41]), and members of the tripartite ATP-independent periplasmic transporters (TRAP-T) (42) (e.g., the TeaABC system [43]). In addition to the typical osmotic induction of the transcription of their structural genes (1,(3)(4)(5)(44)(45)(46), the activities of compatible solute transporters are also frequently enhanced by osmotic stress (6,19,20,32,35,47,48), thereby providing bacterial cells with increased uptake capacity for stress-relieving compounds both under acute and sustained high-osmolarity conditions.…”