Source 1review2000Journal of the American Society of Nephrology
Toolkit/proteoliposome water permeability assay
proteoliposome water permeability assay
Taxonomy: Technique Branch / Method. Workflows sit above the mechanism and technique branches rather than replacing them.
Summary
Highly purified AQP1 protein from human red cells reconstituted with pure phospholipid yielded proteoliposomes of approximately 100 nm in diameter. The change in volume of AQP1 proteoliposomes was compared to liposomes containing no reconstituted protein by measuring quenching of internal carboxyfluorescein.
Usefulness & Problems
Why this is useful
This assay reconstitutes purified AQP1 into phospholipid vesicles and measures water-driven volume changes. The review uses it to show direct water permeability and water selectivity of AQP1.; direct biophysical measurement of channel-mediated water permeability; testing whether purified membrane protein is sufficient for water transport; comparing reconstituted protein vesicles against protein-free liposome controls
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This assay reconstitutes purified AQP1 into phospholipid vesicles and measures water-driven volume changes. The review uses it to show direct water permeability and water selectivity of AQP1.
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direct biophysical measurement of channel-mediated water permeability
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testing whether purified membrane protein is sufficient for water transport
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comparing reconstituted protein vesicles against protein-free liposome controls
Problem solved
It addresses whether the purified protein alone is sufficient to confer water transport and whether that transport is selective.; demonstrating sufficiency and selectivity of a purified water channel
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It addresses whether the purified protein alone is sufficient to confer water transport and whether that transport is selective.
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demonstrating sufficiency and selectivity of a purified water channel
Problem links
demonstrating sufficiency and selectivity of a purified water channel
LiteratureIt addresses whether the purified protein alone is sufficient to confer water transport and whether that transport is selective.
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It addresses whether the purified protein alone is sufficient to confer water transport and whether that transport is selective.
Published Workflows
Objective: Identify and functionally validate molecular water channels responsible for renal and other biologic membrane water permeability.
Why it works: The review describes a progression from candidate identification to heterologous functional testing and then purified-protein reconstitution, allowing investigators to move from correlation to direct demonstration of channel activity and selectivity.
channel-mediated transmembrane water transportheterologous expression for functional testingpurified-protein reconstitution for sufficiency testingbiochemical purificationfunctional reconstitutionexpression cloningXenopus oocyte expressionproteoliposome biophysics
Stages
- 1.Candidate identification from purification, cloning, and tissue distribution(library_design)
The review indicates that water transporters were unknown and candidate molecules had to be recognized before direct functional testing.
Selection: Proteins are prioritized when they are purified, sequenceable or clonable, and abundant in highly water-permeable tissues.
- 2.Heterologous functional assay in Xenopus oocytes(functional_characterization)
The oocyte system provides a low-background functional test for candidate water channel RNAs.
Selection: Candidates are advanced if expression in oocytes increases water permeability relative to water-injected controls.
- 3.Purified-protein reconstitution and direct biophysical confirmation(confirmatory_validation)
This stage tests whether the purified protein alone is sufficient to account for water permeability and whether transport is selective.
Selection: Purified candidate protein is reconstituted into phospholipid vesicles and compared with protein-free liposomes for direct permeability and selectivity measurements.
Steps
- 1.Purify and sequence a candidate membrane protein to enable cDNA cloning
Generate a molecular candidate for water-channel testing.
The review indicates that the molecular identity of water transporters was unknown, so a candidate first had to be isolated and cloned before functional assays could be performed.
- 2.Express candidate water-channel RNA in Xenopus oocytes and test osmotic swellingfunctional assay platform
Determine whether the candidate confers water permeability in a low-background system.
The review explicitly states that oocytes normally exhibit low membrane water permeability, making them useful for functional analysis after a candidate RNA is obtained.
- 3.Reconstitute purified protein into phospholipid vesicles and compare with protein-free liposomesconfirmatory biophysical assay
Test whether the purified protein alone is sufficient for selective water transport.
This confirmatory step follows heterologous expression to rule out dependence on other cellular factors and directly measure permeability and selectivity.
Taxonomy & Function
Primary hierarchy
Technique Branch
Method: A concrete measurement method used to characterize an engineered system.
Mechanisms
fluorescence quenching of internal carboxyfluorescein as a volume readoutmembrane protein reconstitution into proteoliposomesosmotic water flux–driven vesicle volume changeTechniques
Functional AssayTarget processes
No target processes tagged yet.
Implementation Constraints
cofactor dependency: cofactor requirement unknownencoding mode: genetically encodedimplementation constraint: context specific validationoperating role: sensor
It requires highly purified channel protein, pure phospholipid, proteoliposome preparation, and a fluorescence readout based on internal carboxyfluorescein quenching.; requires milligram-scale purified protein; requires phospholipid reconstitution; requires fluorescence-based volume readout such as internal carboxyfluorescein quenching
It does not by itself define native tissue regulation or all possible physiologic functions in vivo.; requires purified protein and reconstitution into phospholipid vesicles
Validation
Cell-freeBacteriaMammalianMouseHumanTherapeuticIndep. Replication
Supporting Sources
Ranked Claims
Biochemical purification and functional reconstitution techniques have been used to identify and characterize multiple transporters.
Using biochemical purification and functional reconstitution techniques, multiple transporters have been identified and characterized.
Proteoliposome reconstitution with fluorescence-based volume measurement can directly quantify water permeability of purified AQP1 and distinguish it from protein-free liposomes.
The change in volume of AQP1 proteoliposomes was compared to liposomes containing no reconstituted protein by measuring quenching of internal carboxyfluorescein.
The Xenopus oocyte expression system is a valuable method for functional analysis of candidate water channel RNAs because oocytes normally have low membrane water permeability.
the Xenopus oocyte expression system has proven to be a valuable method for functional analysis of candidate water channel RNAs, since the oocytes normally exhibit low membrane water permeability
Approval Evidence
1 source1 linked approval claimfirst-pass slug proteoliposome-water-permeability-assay
Highly purified AQP1 protein from human red cells reconstituted with pure phospholipid yielded proteoliposomes of approximately 100 nm in diameter. The change in volume of AQP1 proteoliposomes was compared to liposomes containing no reconstituted protein by measuring quenching of internal carboxyfluorescein.
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method capabilitysupports
Proteoliposome reconstitution with fluorescence-based volume measurement can directly quantify water permeability of purified AQP1 and distinguish it from protein-free liposomes.
The change in volume of AQP1 proteoliposomes was compared to liposomes containing no reconstituted protein by measuring quenching of internal carboxyfluorescein.
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Comparisons
Source-stated alternatives
The review pairs this approach with Xenopus oocyte expression and tissue localization studies.
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The review pairs this approach with Xenopus oocyte expression and tissue localization studies.
Source-backed strengths
supports direct unit permeability measurement; allows comparison to liposomes lacking reconstituted protein; review states it showed AQP1 is water-selective
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supports direct unit permeability measurement
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allows comparison to liposomes lacking reconstituted protein
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review states it showed AQP1 is water-selective
Compared with Langendorff perfused heart electrical recordings
proteoliposome water permeability assay and Langendorff perfused heart electrical recordings address a similar problem space.
Shared frame: same top-level item type
Strengths here: looks easier to implement in practice.
Compared with native green gel system
proteoliposome water permeability assay and native green gel system address a similar problem space.
Shared frame: same top-level item type
Strengths here: looks easier to implement in practice.
proteoliposome water permeability assay and sub-picosecond pump-probe analysis of bacteriorhodopsin pigments address a similar problem space.
Shared frame: same top-level item type
Strengths here: looks easier to implement in practice.
Ranked Citations
- 1.