Membrane proteins and proteomics: Love is possible, but so difficult

Despite decades of extensive research, the large-scale analysis of membrane proteins remains a difficult task. This is due to the fact that membrane proteins require a carefully balanced hydrophilic and lipophilic environment, which optimum varies with different proteins, while most protein chemistry methods work mainly, if not only, in water-based media. Taking this review [Santoni, Molloy and Rabilloud, Membrane proteins and proteomics: un amour impossible? Electrophoresis 2000, 21, 1054-1070] as a pivotal paper, the current paper analyzes how the field of membrane proteomics exacerbated the trend in proteomics, i.e. developing alternate methods to the historical two-dimensional electrophoresis, and thus putting more and more pressure on the mass spectrometry side. However, in the case of membrane proteins, the incentive in doing so is due to the poor solubility of membrane proteins. This review also shows that in some situations, where this solubility problem is less acute, two-dimensional electrophoresis remains a method of choice. Last but not least, this review also critically examines the alternate approaches that have been used for the proteomic analysis of membrane proteins.

💡 Research Summary

The reviewed paper, originally published by Santoni, Molloy and Rabilloud in 2000, provides a comprehensive assessment of why large‑scale membrane‑protein proteomics remains one of the most challenging areas in modern biochemistry. Membrane proteins possess both hydrophilic domains that face the aqueous cellular compartments and hydrophobic trans‑membrane segments that are embedded in the lipid bilayer. This dual nature makes them poorly soluble in the aqueous buffers that dominate classical protein chemistry, and it directly interferes with the two‑dimensional electrophoresis (2‑D PAGE) workflow that has historically underpinned proteomic discovery.

The authors first describe how the requirement for a balanced hydrophilic‑lipophilic environment varies from protein to protein, rendering a single solubilisation cocktail ineffective for the whole membrane‑proteome. Conventional 2‑D PAGE relies on isoelectric focusing (IEF) in the first dimension, a step that demands complete solubilisation and stable charge states. Membrane proteins tend to precipitate or aggregate during IEF, leading to streaking, loss of resolution, and, ultimately, poor representation on the gel. To mitigate this, researchers have introduced a plethora of chaotropes (urea, thiourea), detergents (CHAPS, SDS, non‑ionic and zwitterionic surfactants), and reducing agents, but each additive introduces new variables: altered peptide ionisation, detergent residues that suppress MALDI or ESI signals, and increased sample handling that can cause loss of low‑abundance species.

Because of these complications, the field has gradually shifted away from the classic “2‑D PAGE → MS” pipeline toward alternative front‑ends that place a heavier burden on mass spectrometry. The review catalogues several such strategies. Strong acid or base extractions followed by phase‑partitioning can selectively enrich hydrophobic proteins, yet they risk chemical modification and degradation. Organic‑solvent precipitation (using methanol, isopropanol, or acetone) improves solubility of trans‑membrane helices but may discard peripheral or amphipathic proteins. Non‑ionic surfactants such as deoxycholate or digitonin are compatible with downstream LC‑MS/MS after acid precipitation, but complete removal is labor‑intensive and residual surfactant can still generate background ions. More recent microfluidic “on‑chip” platforms integrate lysis, detergent removal, and direct electrospray, dramatically reducing sample loss and enabling analysis of sub‑microgram quantities, but they require specialized equipment and are not yet widely adopted.

Importantly, the authors argue that the move away from 2‑D electrophoresis is not universal. In cases where membrane proteins are relatively soluble—such as bacterial outer‑membrane proteins, single‑pass receptors, or proteins with large extracytoplasmic domains—2‑D PAGE still offers unmatched separation power, visual mapping of isoforms, and quantitative reproducibility. Adjustments such as narrowing the pH range of IEF strips (e.g., pH 3–6), employing off‑gel IEF, or using immobilized pH gradients with higher detergent concentrations can rescue many problematic proteins. For highly hydrophobic multi‑pass proteins (e.g., GPCRs, ion channels), shotgun LC‑MS/MS after detergent‑based solubilisation or even direct “top‑down” approaches are more efficient.

The review concludes with a set of practical recommendations. Researchers should first characterize the physicochemical profile of their target membrane proteome (hydropathy, predicted trans‑membrane segments, isoelectric points) using bioinformatic tools (TMHMM, Phobius, SOSUI). Based on this profile, they can select an optimal solubilisation cocktail, decide whether to employ 2‑D PAGE or a gel‑free workflow, and plan for rigorous detergent removal before MS. The authors stress that a hybrid strategy—leveraging the visual strengths of 2‑D electrophoresis where feasible while exploiting modern MS‑centric methods for recalcitrant proteins—offers the best chance of comprehensive membrane‑proteome coverage. In this way, the “impossible love” between membrane proteins and proteomics becomes a challenging but achievable partnership.