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EcoSal Plus

Domain 5:

Responding to the Environment

Osmotic Stress

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  • Authors: Karlheinz Altendorf1, Ian R. Booth2, Jay Gralla3, Jörg-Christian Greie4, Adam Z. Rosenthal5, and Janet M. Wood6
  • Editor: James M. Slauch7
  • VIEW AFFILIATIONS HIDE AFFILIATIONS
    Affiliations: 1: Universität Osnabrück, Fachbereich Biologie/Chemie, Abteilung Mikrobiologie, Barbarastrasse 11, D-49069 Osnabrück, Germany; 2: School of Medical Sciences, University of Aberdeen, Institute of Medical Sciences, Foresterhill, Aberdeen, AB25 2ZD, United Kingdom; 3: Department of Chemistry and Biochemistry and the Molecular Biology Institute, University of California, Los Angeles, PO Box 951569, Los Angeles, CA 90095; 4: Universität Osnabrück, Fachbereich Biologie/Chemie, Abteilung Mikrobiologie, Barbarastrasse 11, D-49069 Osnabrück, Germany; 5: Department of Chemistry and Biochemistry and the Molecular Biology Institute, University of California, Los Angeles, PO Box 951569, Los Angeles, CA 90095; 6: Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON, Canada N1G 2W1; 7: The Schoold of Molecular and Cellular Biology, University of Illinois at Urbana-Champaign, Urbana, IL
  • Received 05 February 2009 Accepted 14 May 2009 Published 19 November 2009
  • Address correspondence to Janet M. Wood jwood@uoguelph.ca.
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  • Abstract:

    and encounter osmotic pressure variations in natural environments that include host tissues, food, soil, and water. Osmotic stress causes water to flow into or out of cells, changing their structure, physics, and chemistry in ways that perturb cell functions. and limit osmotically induced water fluxes by accumulating and releasing electrolytes and small organic solutes, some denoted compatible solutes because they accumulate to high levels without disturbing cell functions. Osmotic upshifts inhibit membrane-based energy transduction and macromolecule synthesis while activating existing osmoregulatory systems and specifically inducing osmoregulatory genes. The osmoregulatory response depends on the availability of osmoprotectants (exogenous organic compounds that can be taken up to become compatible solutes). Without osmoprotectants, K accumulates with counterion glutamate, and compatible solute trehalose is synthesized. Available osmoprotectants are taken up via transporters ProP, ProU, BetT, and BetU. The resulting compatible solute accumulation attenuates the K glutamate response and more effectively restores cell hydration and growth. Osmotic downshifts abruptly increase turgor pressure and strain the cytoplasmic membrane. Mechanosensitive channels like MscS and MscL open to allow nonspecific solute efflux and forestall cell lysis. Research frontiers include (i) the osmoadaptive remodeling of cell structure, (ii) the mechanisms by which osmotic stress alters gene expression, (iii) the mechanisms by which transporters and channels detect and respond to osmotic pressure changes, (iv) the coordination of osmoregulatory programs and selection of available osmoprotectants, and (v) the roles played by osmoregulatory mechanisms as and survive or thrive in their natural environments.

  • Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5

Key Concept Ranking

Bacteria and Archaea
0.811354
KdpDE Two-Component Regulatory System
0.44199157
Gene Expression and Regulation
0.4386641
Chemicals
0.37281206
Major Facilitator Superfamily
0.37166303
0.811354

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/content/journal/ecosalplus/10.1128/ecosalplus.5.4.5
2009-11-19
2017-05-29

Abstract:

and encounter osmotic pressure variations in natural environments that include host tissues, food, soil, and water. Osmotic stress causes water to flow into or out of cells, changing their structure, physics, and chemistry in ways that perturb cell functions. and limit osmotically induced water fluxes by accumulating and releasing electrolytes and small organic solutes, some denoted compatible solutes because they accumulate to high levels without disturbing cell functions. Osmotic upshifts inhibit membrane-based energy transduction and macromolecule synthesis while activating existing osmoregulatory systems and specifically inducing osmoregulatory genes. The osmoregulatory response depends on the availability of osmoprotectants (exogenous organic compounds that can be taken up to become compatible solutes). Without osmoprotectants, K accumulates with counterion glutamate, and compatible solute trehalose is synthesized. Available osmoprotectants are taken up via transporters ProP, ProU, BetT, and BetU. The resulting compatible solute accumulation attenuates the K glutamate response and more effectively restores cell hydration and growth. Osmotic downshifts abruptly increase turgor pressure and strain the cytoplasmic membrane. Mechanosensitive channels like MscS and MscL open to allow nonspecific solute efflux and forestall cell lysis. Research frontiers include (i) the osmoadaptive remodeling of cell structure, (ii) the mechanisms by which osmotic stress alters gene expression, (iii) the mechanisms by which transporters and channels detect and respond to osmotic pressure changes, (iv) the coordination of osmoregulatory programs and selection of available osmoprotectants, and (v) the roles played by osmoregulatory mechanisms as and survive or thrive in their natural environments.

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Figures

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Figure 1

Osmotic upshifts stimulate K uptake, putrescine efflux, and the accumulation of glutamate as K counterion. Glutathione, MOPS (3-[-morpholino]propanesulfonate), and other anions can also accumulate in cells under osmotic stress. Later, trehalose is synthesized and potassium glutamate levels fall. Uptake of available osmoprotectants attenuates those responses. Osmoprotectant choline is taken up and oxidized to glycine betaine. Osmoprotectants glycine betaine, proline, proline betaine, taurine, pipecolate, ectoine, and carnitine are transported into the cytoplasm where they accumulate unmodified as “compatible solutes.” The mechanisms responsible for solute accumulation and release are listed in Table 3 and illustrated in Fig. 2 .

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Figure 2

Aquaporin AqpZ mediates transmembrane water flux. K transporter TrkAE(G/H) mediates K accumulation in response to high osmotic pressure (TrkE is also known as SapD); K transporter KdpFABC contributes to a lesser extent. KdpDE is a two-component regulatory system that controls transcription in response to K supply and osmotic stress. Suppression of glutamate catabolism leads to its accumulation as K counterion. At high osmotic pressure trehalase (TreA) hydrolyzes extracellular trehalose. OtsA and OtsB mediate trehalose synthesis. Expression of (encoding trehalose-specific Enzyme IIBC) and (encoding trehalose-6-phosphate phosphatase) is repressed. Transporters ProP, ProU, BetT, and BetU mediate organic osmolyte accumulation at high osmotic pressure. Their properties are summarized in Table 4 . Enzymes BetA and BetB mediate glycine betaine synthesis from choline. Mechanosensitive channels MscL and MscS mediate solute efflux in response to decreasing osmotic pressure. Factors regulating transcription of the genes encoding these systems are listed in Table 3 .

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Figure 3

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Figure 4

(A) LacY activity decreases as ProP activity increases in membrane vesicles under osmotic stress. Membrane vesicles containing both LacY and ProP were prepared, then proline (circles) and lactose (triangles) uptake activities were measured in K phosphate supplemented with sucrose to adjust the osmolality (and hence, the water activity). (B) Impacts of ΔΨ on the osmotic activation of ProP in proteoliposomes. The impact of osmolality on ProP-His6 activity in proteoliposomes was determined with ΔpH fixed at 1 unit (−59 mV) as ΔΨ varied from 0 to −137 mV. The initial rate of proline uptake via ProP-His6 is plotted versus osmolality, with ΔΨ at −137 mV (filled circles), −120 mV (squares), −100 mV (triangles), and 0 (open circles). The initial rate of proline uptake at high osmolality increases with ΔΨ because the proton motive force powers H-proline symport. In addition, the initial rate of proline uptake at low osmolality decreases as the magnitude of ΔΨ increases, showing that ΔΨ is required for the osmoregulation of ProP activity.

These figures were reproduced with permission from Culham et al. (2008), 8176–8185, ©2008, American Chemical Society.

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Figure 5

ProP is a member of the Major Facilitator Superfamily ( 196 ) that catalyzes osmolality-dependent H-osmoprotectant symport ( 95 ). (A) ProP structure. ProP is a 500-residue protein with 12 transmembrane α-helices, a cytoplasmic N terminus, and an extended, cytoplasmic C-terminal domain ( 192 ). The illustrations show a homology model for residues 4 to 236 and 246 to 452 of ProP (left, Protein Data Bank [PDB] ID code 1Y8S [ 192 ]) and an NMR structure of the homodimeric, antiparallel α-helical coiled-coil formed by a peptide replica of ProP residues 468 to 497 (right, PDB ID code 1R48 [ 197 , 198 ]). The structures of the intervening peptides (residues 237 to 245 and 453 to 467) are unknown. Residues in ribbon format are yellow (nonpolar), green (polar), red (acidic), or blue (basic). Primes denote residues in the second copy of the homodimeric coiled-coil. The homology model (left) is based on the crystal structure of anion antiporter GlpT (PDB ID code 1PW4). ProP and other symporters are believed to function via an alternating access mechanism in which substrate binding sites alternately face the periplasm and cytoplasm ( 199 , 200 ).

Similarly to the GlpT structure, the ProP model would represent the “inward facing” conformation of the transporter. The black oval encircles the cluster of intramembrane polar residues within the N-terminal helix bundle that may be involved in H-osmoprotectant symport. No polar residues are found within the intramembrane portion of the C-terminal helix bundle in this model. Single-cysteine variants facilitate structure-function analysis of ProP ( 190 , 191 , 201 , 202 ). A bifunctional alkylating reagent covalently cross-linked dimers of ProP variants with cysteine only at position 419, 420, 422, 439, or 480 in vivo. Variants with cysteine at other positions, including 133, 241, or 473, were not cross-linked. This and other evidence indicates that antiparallel coiled-coils link ProP monomers in vivo ( 191 , 201 , 202 ). Arg 488 (left, space-filling representation with CPK color), embedded in the hydrophobic coiled-coil interface, stabilizes the antiparallel structure by extending to the hydrophilic surface to interact with acidic residues of the opposite strand ( 107 , 198 , 203 ). (B) ProP function. The homology model ( 192 ) is used to illustrate the locations of functionally important residues in ProP. Alkylating agents have been used to probe the solvent exposure of cysteines in single-cysteine ProP variants ( 190 , 191 , 192 ). Like ProP activity, the reactivities of residues C59 (periplasmic loop 1) and C415-C418 (periplasmic loop 6) increased with osmolality. The reactivities of other periplasmic residues did not. Amino acid replacements Y44M, S62C, and Y423C render ProP activity osmolality-insensitive ( 190 , 191 , 192 ). Replacement Y44M also rendered alkylation of C59 or C415 osmolality-insensitive, as did dissipation of the proton motive force ( 190 ). According to the structural model, Y44 caps the cluster of polar residues within the N-terminal bundle of ProP that may be involved in H-osmoprotectant symport (ball-and-stick structures encircled by black oval).

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Figure 6

(A) Crystal structures of the closed (nonconducting) forms of MscL (a) and MscS (b) from ( 252 , 253 ) and ( 253 , 282 ). Color coding: (a) MscL: amino-terminal helix (red), TM1 (blue), TM2 (yellow), carboxy-terminal helix (green). (b) MscS: TM1-TM2 (yellow), pore-lining helix TM3a (blue), tensor helix TM3b (yellow), and carboxy-terminal domain (green). Although the color coding implies functional similarities between the domains sharing the same color in the two proteins, this cannot be used too literally. Thus, although TM2 of MscL and TM1-TM2 of MscS are the major lipid facing domains, mutations that affect the response to changes in lateral tension are found in both TM1 and TM2 of MscL. (B) Model for the structural transition for MscS. The model is based on the crystal structures of MscS in the nonconducting (a) ( 253 , 282 ) and the conducting state ( 283 ) (b). The color coding of the helices is as in panel A. A cartoon representation of the movements required to attain the open, conducting state (c) shows that the TM3a helices rotate clockwise (as viewed from the periplasm) and move out from the axis of the pore as these helices straighten and separate.

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Figure 7

Two views are shown for each of the channels: the side view and the view from the periplasm down the axis of the channel. Some of the residues that are critical for function are highlighted. (a, b) MscL Val21 (equivalent of Val23 in MscL [MscL from ]), is blue; the positions equivalent to MscL residues Gly24, Gly26, and Gly30 are red. Residues that, when mutated to Asn or to Trp, cause loss of channel function, which is believed to be the result of altered interaction with lipid headgroups ( 286 , 287 ), are green (TM1) or yellow (TM2). (c) A side view of MscS is shown in the closed state in which the seal residues (L105 and L109) are blue. The critical Gly residues (101, 104, and 108), which are central to the closed-open transition, are orange, and Ala98 and Ala106 are green. The Ala102 residue that is critical for the stable open state of the channel is shown in vermillion. Note that in this representation TM1-TM2 of one subunit has been removed to allow sight of the interactions between Gly and Ala in the TM3a helices. The residues forming the carboxy-terminal domain have been removed for clarity. (d) MscS viewed from the periplasm. The missing TM1-TM2 segment has been restored and is pink to enable the viewer to see that TM3b of one subunit (pink) interacts with TM1–2 (silver) of the next subunit (clockwise). The other five subunits are shown in dark gray. Note that in MscL the amino terminus of the pore-lining TM1 helix is cytoplasmic, whereas in MscS the amino terminus of the pore-lining TM3a helix is periplasmic.

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Tables

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Table 1

The vocabulary of bacterial osmotolerance, osmosensing, and osmoregulation

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
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Table 2

Quantities relevant to osmotic calculations

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
Generic image for table
Table 3

Osmoregulatory systems of and

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5
Generic image for table
Table 4

Osmoprotectant transporters of and

Citation: Altendorf K, Booth I, Gralla J, Greie J, Rosenthal A, Wood J. 2009. Osmotic Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.5

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