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Two-dimensional gel electrophoresis in proteomics: A tutorial
Thierry Rabilloud, Cécile Lelong
TL;DR
The paper addresses how two-dimensional electrophoresis fits into modern proteomics despite its established limitations. It presents a tutorial of the workflow, constraints, detection stage, applications, and future scope, emphasizing intact-protein separation and quantitative spot selection. The technique remains useful for robust analysis of large sample series and for studying post-translational modifications, while membrane proteins and expression dynamics constrain its use.
Problem
Two-dimensional electrophoresis has distinct proteomic features but also important constraints, requiring guidance on its workflow, applications, and appropriate current and future uses.
Method
The paper provides a tutorial covering sample preparation, sequential separations, interfacing, protein detection, image analysis, limitations, applications, and future directions.
Results
2D gel-based proteomics supports quantitative spot selection, reduces downstream mass-spectrometry workload, and enables detection of modification-associated migration shifts.
Takeaways & Limitations
2D electrophoresis remains suited to robust quantitative analysis of large sample series and high-resolution separation of intact proteins with associated post-translational modifications.
Takeaways & Limitations
In eukaryotic samples, membrane proteins and broad protein-expression dynamics limit the proteins that 2D gels can analyze effectively.
Abstract
from arXiv · showhide
Two-dimensional electrophoresis of proteins has preceded, and accompanied, the birth of proteomics. Although it is no longer the only experimental scheme used in modern proteomics, it still has distinct features and advantages. The purpose of this tutorial paper is to guide the reader through the history of the field, then through the main steps of the process, from sample preparation to in-gel detection of proteins, commenting the constraints and caveats of the technique. Then the limitations and positive features of two-dimensional electrophoresis are discussed (e.g. its unique ability to separate complete proteins and its easy interfacing with immunoblotting techniques), so that the optimal type of applications of this technique in current and future proteomics can be perceived. This is illustrated by a detailed example taken from the literature and commented in detail. This Tutorial is part of the International Proteomics Tutorial Programme (IPTP 2).
1. Historical Background
Two-dimensional electrophoresis emerged by coupling SDS zone electrophoresis and denaturing isoelectric focusing, which separate proteins by molecular mass and isoelectric point. An early 1974 report attracted little attention because sample inclusion was difficult and Coomassie staining was insensitive.
- In the early 1970s, SDS zone electrophoresis and denaturing isoelectric focusing provided independent protein separations by molecular mass and isoelectric point.
- The first successful coupling of these techniques was reported in 1974.
- The 1974 report received little attention because sample inclusion in the IEF gel was difficult and Coomassie blue staining was relatively insensitive.
- Later mass spectrometry made protein characterization faster, more sensitive, and deeper than Edman sequencing after 2D electrophoresis.
2. Basic concepts
Two-dimensional electrophoresis combines sequential isoelectric focusing and SDS electrophoresis with sample preparation, interfacing, and protein detection. Its strengths include reproducible quantitative analysis, selective downstream mass spectrometry, immunoblot compatibility, and sensitivity to post-translational modifications, while sample solubility and precipitation remain important constraints.
- Workflow: 2D electrophoresis is a five-step workflow comprising sample preparation, first separation, interfacing, second separation, and protein detection.The classical order is isoelectric focusing followed by SDS electrophoresis, which is favored for economical and technical reasons.
- Sample preparation: High field strengths in isoelectric focusing require low ionic strength, complicating disruption of protein–nucleic acid interactions during sample preparation.Fields up to 170V/cm are common in isoelectric focusing, compared with 15V/cm in SDS electrophoresis; residual interactions can produce artifacts.
- Isoelectric focusing: Sample application and voltage programming involve trade-offs: dispersed loading can improve resolution but cause protein losses, while gradual voltage increases limit salt-induced Joule heating.Immobilized-gradient strip rehydration permits substantial loading but induces severe protein losses and fails completely for basic pH gradients; a typical moderate phase is 15V/cm for 3 hours.
- Isoelectric focusing: Isoelectric precipitation is an essential drawback because proteins have minimal solubility and no electrostatic repulsion at their pI, making membrane-protein analysis particularly unsuccessful.The problem worsens as intrinsic protein solubility decreases.
- Isoelectric focusing: Immobilized pH gradients have made 2D gels easier, more reproducible, and more tunable through nonlinear, basic, and narrow pH-gradient designs.These gradients also reduce deformation compared with earlier carrier-ampholyte systems.
- Protein detection and image analysis: Image analysis quantifies multiple gel images and selects spots for downstream analysis, so parallel gel runs, experimental design, and statistical care are critical.In an illustrative ten-sample comparison, 2D gels might reduce mass-spectrometry work from 200 hours to at most 20 hours by selecting about 20 spots, without being intrinsically more productive.
- Applications: 2D gels are useful for large sample series, bacterial proteomics, immunoproteomics, and post-translational-modification studies because they support quantitative comparison, antibody probing, and modification-dependent spot shifts.Phosphorylation, glycosylation, glutathionylation, and cleavage can alter pI and/or molecular weight, enabling unsupervised detection followed by mass-spectrometric identification.
4. Worked example
The worked example examines oxidative stress response, where stress produces quantitative protein changes and direct protein modifications. It illustrates that modified protein forms can sometimes be visualized even when modified peptides would be missed by direct peptide-based proteomics.
- Oxidative stress response is examined through quantitative protein variations and direct protein modifications.
- Modified protein forms can sometimes be visualized at the protein level despite modified peptides being missed by direct peptide-based proteomics.
5. Current limitations and working limits
Two-dimensional gel-based proteomics is limited by membrane-protein analysis and by the uneven expression dynamics of cellular proteins. Fractionation, including organelle analysis, can reduce sample complexity and provide access to lower-abundance proteins.
- Membrane proteins are a documented limitation of two-dimensional gel analysis.
- In budding yeast, 130 gene products, representing 2% of the genome, account for 50% of protein mass.
- The top 10% most expressed yeast genes produce 75% of protein mass, while the low-expressed two-thirds produce 10%.
- Fractionation lowers sample complexity and can improve access to low-abundance proteins, including poorly annotated proteins in cell organelles.
6. Future directions
2D electrophoresis is a mature, flexible technique whose future value lies less in improving standalone gel performance than in targeted upstream fractionation and applications where intact-protein separation matters. Its high-resolution separation of complete proteins with associated post-translational modifications supports continued use in proteomics.
- 6. Future directions: 2D electrophoresis is mature, flexible, and offers tunable resolution, leaving little expected gain from improving pure gel performance alone.The paper describes more than three decades of development and notes that resolution can be tuned with established methods.
- 6. Future directions: Future progress is expected from applying more 2D gels to cell fractions and developing more robust fractionation techniques.The authors identify upstream fractionation as a decisive direction for improving 2D gel-based proteomics.
- 6. Future directions: The technique is expected to remain useful where its intact-protein and modification-resolving capabilities add value compared with other proteomics setups.The paper recommends using 2D gels where they provide the most added value relative to alternative proteomics approaches.
- 6. Future directions: 2D gel-based proteomics remains valuable because it separates intact proteins at high resolution together with their associated post-translational modifications.This feature is presented as especially important for determining protein qualities, including post-translational modifications.