Methods for Detecting Protein Phosphorylation

Introduction

Protein kinases transfer phosphate groups from ATP to serine, threonine, or tyrosine residues on protein peptide substrates, directly affecting the activity and function of the target. Radiolabel studies suggest that approximately 30% of proteins in eukaryotic cells are subject to phosphorylation.^{1, 2} This crucial post-translational modification regulates a broad range of cellular activities including the cell cycle, differentiation, metabolism, and neuronal communication. In addition, abnormal phosphorylation events are implicated in many disease states. When assessing phosphorylation, the method of choice may vary depending on many factors including the specific question being asked and availability of specialized equipment or reagents. This review provides a brief description of several methodologies currently used and addresses some of the benefits and drawbacks associated with each.

Table of Contents

Kinase Activity Assays

Protein kinases are often common elements in multiple signaling networks influencing numerous downstream effectors responsible for a biological response. Thus, assessing the activity of a single specific kinase may provide valuable information on parallel pathways.³ Kinase activity within a biological sample is commonly measured in vitro by incubating the immunoprecipitated kinase with an exogenous substrate in the presence of ATP. Measurement of the phosphorylated substrate by a specific kinase can be assessed by several reporter systems including colorimetric, radioactive, or fluorometric detection.⁴ Additionally, R&D Systems offers a non-radioactive Universal Kinase Activity Kit that allows for the quantification of kinase activity for any kinase that produces ADP. Although information can be obtained regarding the actions of a specific kinase, assessing enzyme activity in cellular extracts only provides a glimpse of the signaling landscape. Little is revealed about the proteins being modified, and in vitro activity assays do not address the role of potential endogenous phosphatase activity. Direct detection of phosphorylated proteins can provide a more detailed analysis of the cellular response to an external stimulus, as identification of a phosphopeptide provides information regarding the expression and the functional state of that protein.

Phospho-Specific Antibody Development

A classical method of directly measuring protein phosphorylation involves the incubation of whole cells with radiolabeled ³²P-orthophosphate, the generation of cellular extracts, separation of proteins by SDS-PAGE, and exposure to film.^{2, 5} This labor-intensive method requires many multi-hour incubations and the use of radioisotopes. Other traditional methods include 2-dimensional gel electrophoresis, a technique that assumes phosphorylation will alter the mobility and isoelectric point of the protein.

In light of these laborious methods, the development of phosphorylation-dependent antibodies was a welcome event for researchers. In 1981, the first documented phospho-antibody was produced in rabbits immunized with benzonyl phosphonate conjugated to keyhole limpet hemocyanin (KLH). This antibody broadly recognized proteins containing phosphotyrosine.⁶ Ten years later, phosphorlyation state-specific (phospho-specific) antibodies were developed by immunizing rabbits with synthetic phosphopeptides representing the amino acid sequence surrounding the phosphorylation site of the target protein.⁷ The immune sera was applied to a peptide affinity column to generate a highly specific immunoreagent. The availability of phospho-specific antibodies has opened the door for the improvement of traditional methods as well as the development of new immunoassay techniques. The main caveat in utilizing phospho-specific antibodies in any technique is that successful detection is dependent on the specificity and affinity of the antibody for the phospho-protein of interest.

Simple Western Capillary Western Assays

Simple Western™ is a next-generation capillary immunoassay technology that performs automated Western blot analysis inside a benchtop instrument like Jess™, Abby™, Peggy Sue™, Sally Sue™, and NanoPro™ 1000. Compared to traditional methods like Western blot, Simple Western instruments can detect and quantify phosphorylated isoforms of target proteins with high specificity, reproducibility, and sensitivity. The Stellar NIR/IR Modules for Jess lead the industry in fluorescence detection sensitivity for Western blotting workflows, with 100X greater sensitivity than the closest competing fluorescence imaging technology. As an open immunoassay platform using conventional antibodies for detection, Simple Western assays can detect phosphorylated proteins in two ways:

Western Blot

The Western blot is the most common method used for assessing the phosphorylation state of a protein, and most cell biology laboratories possess the equipment necessary to perform these experiments. Following separation of the biological sample with SDS-PAGE and subsequent transfer to a membrane (usually PVDF or nitrocellulose), a phospho-specific antibody can be used to identify the protein of interest (View phospho-specific antibodies for Western blot analysis). The typical Western blot protocol eliminates the hazards and waste disposal requirements associated with the use of radioisotopes.

Enzyme-Linked Immunosorbent Assay (ELISA)

The ELISA has become a powerful method for measuring protein phosphorylation. ELISAs are more quantitative than Western blotting and show great utility in studies that modulate kinase activity and function. The format for this microplate-based assay typically utilizes a capture antibody specific for the desired protein, independent of the phosphorylation state. The target protein, either purified or as a component in a complex heterogeneous sample such as a cell lysate, is then bound to the antibody-coated plate. A detection antibody specific for the phosphorylation site to be analyzed is then added. These assays are typically designed using colorimetric or fluorometric detection. The intensity of the resulting signal is directly proportional to the concentration of phosphorylated protein present in the original sample (Figure 4). The phospho-specific ELISA technique confers several advantages over more traditional immunoblotting in the measurement of protein phosphorylation. First, results are easily quantifiable by utilizing a calibrated standard. Second, high specificity is possible due to the use of two antibodies specific for the target protein employed together in the sandwich format. Thirdly, the higher sensitivity often accomplished using ELISAs allows for smaller sample volumes and the detection of low abundance proteins. Finally, the microplate-based format allows for much higher throughput than traditional Western blotting (View Surveyor IC  ELISA Kits). ELISAs generally provide an indirect measurement of kinase activity. However, variations in the technique described above use an immobilized capture antibody, substrate, and a phospho-substrate detection method for more direct measurements of kinase activity.

Cell-Based ELISA

Although in vitro biochemical kinase assays such as the typical sandwich ELISA are routinely used for hypothesis testing and drug screening, they cannot replicate the intracellular environment. Analyzing protein phosphorylation within intact cells may more accurately represent the status of specific signaling networks. Several immunoassays enabling the measurement of protein phosphorylation in the context of a whole cell have recently been developed. The cells are stimulated, fixed, and blocked in the same well. Phospho-specific antibodies are used to assess phosphorylation status using fluorometric (Figure 5) or colorimetric detection systems. Furthermore, the phospho-protein and total protein are simultaneously detected in the same microplate well. Therefore, signals derived from the target protein can be normalized to that of the second protein, correcting for well-to-well variations and allowing phospho-protein levels to be accurately assessed and compared across multiple samples, similar to using phospho-specific and total protein antibodies in a traditional immunoblot. These assays bypass the need for the creation of cell lysates and are therefore more amenable to high throughput analyses (View Cell-Based ELISA Kits).

Intracellular Flow Cytometry and ICC/IHC

The traditional techniques of intracellular flow cytometry and immunocytochemistry/immunohistochemistry (ICC/IHC) are powerful tools for detecting phosphorylation events.^{8, 9} Flow cytometry uses a laser to excite the fluorochrome used for antibody detection. Filter sets and fluorochromes with non-overlapping spectra must be carefully chosen when assessing multiple proteins in the same cell. Flow cytometry is advantageous because it allows for rapid, quantitative, single cell analysis (Figure 6).¹⁰ Proteins can be detected in a specific cell type within a heterogeneous population via cell surface marker phenotyping without the need to physically separate the cells. In this way, a small, rare population of cells may be analyzed without concern for cell loss or altered protein expression that may occur during a cell-sorting process (View phospho-specific antibodies for flow cytometry).

ICC generally refers to protein detection by microscopy in cultured cells, while IHC refers to protein detection in intact tissue sections. Like flow cytometry, these techniques allow for the assessment of multiple proteins within a cell or tissue provided that adequate attention is given to avoid overlapping fluorescence spectra or color. Both fluorescent and colorimetric detection techniques are commonly used (Figure 7). In contrast to other formats for monitoring phosphorylation, ICC is usually the method of choice for determining intracellular localization (View phospho-specific antibodies for ICC/IHC). Both flow cytometry and ICC/IHC require high-affinity and high-specificity antibodies, blocking steps, controls, and antibody titration to eliminate ambiguous results due to non-specific binding.

Detection of phospho-proteins by flow cytometry and ICC require that the protein is stable and accessible to the antibody. Cells are usually stimulated and fixed with formaldehyde or paraformaldehyde to cross-link the phospho-proteins and stabilize them for analysis. The fixed cells must then be permeabilized to allow for entry of phospho-specific antibodies into the cells. Different permeabilization techniques are often useful for various subcellular locations. A mild detergent will allow for detection of cytoplasmic proteins, while alcohol may be required for antibody access to nuclear proteins. Alcohol permeabilization may also enhance phospho-protein detection using peptide specific antibodies due to the denaturing property of alcohol.¹⁰ For more detailed ICC/IHC protocols, please visit our website at www.rndsystems.com/protocol-types/iccihc-protocols.

Mass Spectrometry

A comprehensive assessment of protein phosphorylation (phosphoproteomics) in complex biological samples, such as cell lysates, is important for understanding phosphorylation-based signaling networks. Large-scale phospho-protein analysis in complex protein mixtures involves identification of phospho-proteins and phosphopeptides and sequencing of the phosphorylated residues. Mass spectrometry (MS) techniques are useful tools for these tasks. Although MS can be used with excellent sensitivity and resolution to identify a single protein, there are several inherent difficulties for the analysis of phospho-proteins. First, signals from phosphopeptides are generally weaker, as they are negatively charged and poorly ionized by electrospray MS, which is performed in the positive mode.¹¹ Second, it can be difficult to observe the signals from low-abundance phospho-proteins of interest in the high-background of abundant non-phosphorylated proteins. To overcome these drawbacks, several enrichment strategies for phospho-protein analysis by MS have been developed including immobilized metal affinity chromatography (IMAC),¹² phosphospecific antibody enrichment,¹³ chemical-modification-based methods such as beta-elimination of phospho-serine and -threonine,¹⁴ and replacement of the phosphate group with biotinylated moieties.¹⁵

Multi-Analyte Profiling

Mass spectrometric techniques such as collision-induced dissociation (CID) and electron transfer dissociation (ETD) provide comprehensive parallel analysis of peptide sequences and post-translational modifications such as phosphorylation.¹⁶ These techniques are labor-intensive, and strategies for comprehensive phosphorylation analysis may not be needed if particular pathways are of primary interest. This has led to the development of several novel methods for measuring protein phosphorylation of multiple analytes simultaneously. In general, these involve the use of phospho-specific antibodies and include microplate-based (View Proteome Profiler 96 Antibody Arrays) and membrane-based (View Proteome Profiler Antibody Arrays) detection formats (Figure 8-12).^17,18 The obvious benefit of these assays is that throughput capability is greatly enhanced by bypassing the need for running multiple individual Western blots or traditional ELISA-based assays. These techniques are also known for providing more data while requiring very little sample volume. In trade, protein profiling assays are typically recognized as being less sensitive than their more conventional counterparts due to potential antibody cross-reactivity.

Conclusion

Assessing protein phosphorylation is often an essential component of the cell biologist's repertoire for understanding intracellular factors underlying cellular activities. Given the important role kinases play, it is critical for researchers to have quality tools for measuring protein phosphorylation and/or kinase activity. Each technique excels in different contexts, and care must be taken to choose the method that best fits the experimental design. This review provides a brief glimpse of several of the most widely used methods for assessing protein phosphorylation. Because of a growing demand, methodologies continue to improve, bringing researchers closer to understanding these complex and important processes that ultimately control cellular function.