A general method for the rapid characterization of tyrosine-phosphorylated proteins by mini two-dimensional gel electrophoresis

Characterization of Phosphorylated Proteins Using Mass Spectrometry

Phosphorylation is arguably the most important post-translational modification that occurs within proteins. Phosphorylation is used as a signal to control numerous physiological activities ranging from gene expression to metabolism. Identifying phosphorylation sites within proteins was historically a challenge as it required either radioisotope labeling or the use of phospho-specific antibodies. The advent of mass spectrometry (MS) has had a major impact on the ability to qualitatively and quantitatively characterize phosphorylated proteins. In this article, we describe MS methods for characterizing phosphorylation sites within individual proteins as well as entire proteome samples. The utility of these methods is illustrated in examples that show the information that can be gained using these MS techniques.

2D-DIGE: comparative proteomics of cellular signalling pathways

Two-dimensional (2-D) gel electrophoresis concerted with protein identification by mass spectrometry (MS) is an extremely powerful method for comparative expression profiling of complex protein samples such as cell lysates. The highly resolutive 2-D electrophoresis allows the separation of heterogeneous protein samples on the basis of isoelectric point (pI), molecular mass (Mr), solubility, and relative abundance ((1) J Biol Chem 250: 4007-4021, 1975; (2) Electrophoresis 14: 1067-1073, 1993). Consequently, it provides a comprehensive view of a proteome state ((3) Electrophoresis 21: 1037-1053, 2000), where variations in protein expression levels, isoforms, or post-translational modifications (e.g. phosphorylation) can be highlighted and investigated ((4) Electrophoresis 21: 2196-2208, 2000). Furthermore, this allows the identification of biological markers that characterize a specific physiological or pathological background of a cell or a tissue ((5) Proteomics 1: 397-408, 2001; (6) J Bacteriol 179: 7595-7599, 1997). In this way one can compare the effects of a stimulus or drug on cells or tissue, or more importantly, analyse the effects of disease on the expression level of proteins. Relatively recently, conventional 2-D gel electrophoresis has been combined with protein labelling strategies using up to three different fluorescent dyes to allow comparative analysis of different protein samples within a single 2-D gel platform. In this technique, termed differential in-gel electrophoresis (DIGE), samples are labelled separately then combined and run on the same 2D gel minimizing experimental variation and greatly facilitating spot matching. When three CyDyes (Cy2, Cy3, and Cy5) have been used, three images of the gel are captured then superposed to localize the differentially regulated spots on the 2-D gel using image analysis software. This is an extremely powerful tool in comparative proteomics as these dyes provide a linear response to protein concentration up to five orders of magnitude and great sensitivity with detection down to 125 pg of a single protein, which is less than needed for MS identification. In this chapter, we describe the basic methods for protein labelling, optimization of the isoelectrofocusing parameters for the first dimension (where proteins are separated according to their isoelectric point (pI)), sodium-dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) separation for the second dimension (based on molecular weight (MW)), and different post-staining protocols of the 2-D gel and protein preparation for mass spectrometry identification.

Methods and approaches for the comprehensive characterization and quantification of cellular proteomes using mass spectrometry

Proteomics has been proposed as one of the key technologies in the postgenomic era. So far, however, the comprehensive analysis of cellular proteomes has been a challenge because of the dynamic nature and complexity of the multitude of proteins in cells and tissues. Various approaches have been established for the analyses of proteins in a cell at a given state, and mass spectrometry (MS) has proven to be an efficient and versatile tool. MS-based proteomics approaches have significantly improved beyond the initial identification of proteins to comprehensive characterization and quantification of proteomes and their posttranslational modifications (PTMs). Despite these advances, there is still ongoing development of new technologies to profile and analyze cellular proteomes more completely and efficiently. In this review, we focus on MS-based techniques, describe basic approaches for MS-based profiling of cellular proteomes and analysis methods to identify proteins in complex mixtures, and discuss the different approaches for quantitative proteome analysis. Finally, we briefly discuss novel developments for the analysis of PTMs. Altered levels of PTM, sometimes in the absence of protein expression changes, are often linked to cellular responses and disease states, and the comprehensive analysis of cellular proteome would not be complete without the identification and quantification of the extent of PTMs of proteins.

Application of Relative Warp Analysis to the Evaluation of Two-Dimensional Gels in Proteomics: Studying Isoelectric Point and Relative Molecular Mass Variation

We propose a geometric-morphometrics method (relative warp analysis) to be used in proteomic comparisons. This approach was applied to a dataset from a comparison between 5 controls and 5 patients with colorectal cancer disease published elsewhere. The spots in the 2-D maps were used as landmarks in a morphometric study, and the differences in shape (spot distribution) among them were obtained. The shape variables were used to compare controls and patients. These components mostly ignore random or experimental effects affecting all the proteins in any of the two dimensions studied. Furthermore, the method allows the researcher to find those proteins which contributed the most to the local shape component detected. Applying this approach, we detected variations in the position (isoelectric point and/or relative molecular mass) of some spots that may reflect differences in the amino acidic sequence or post-translational modifications.

Proteomic approaches to the diagnosis, treatment, and monitoring of cancer

The field of proteomics holds promise for the discovery of new biomarkers for the early detection and diagnosis of disease, molecular targets for therapy and markers for therapeutic efficacy and toxicity. A variety of proteomics approaches may be used to address these goals. Two-dimensional gel electrophoresis (2D-PAGE) is the cornerstone of many discovery-based proteomics studies. Technologies such as laser capture microdissection (LCM) and highly sensitive MS methods are currently being used together to identify greater numbers of lower abundance proteins that are differentially expressed between defined cell populations. Newer technologies such as reverse phase protein arrays will enable the identification and profiling of target pathways in small biopsy specimens. Surface-enhanced laser desorption/ionization time-of-flight (SELDI-TOF) analysis enables the high throughput characterization of lysates from very few tumor cells or body fluids and may be best suited for diagnosis and monitoring of disease. Such technologies are expected to supplement our arsenal of mRNA-based assays, and we believe that in the future, entire cellular networks and not just a single deregulated protein will be the target of therapeutics and that we will soon be able to monitor the status of these pathways in diseased cells before, during and after therapy.

An improved formulation of SYPRO Ruby protein gel stain: comparison with the original formulation and with a ruthenium II tris (bathophenanthroline disulfonate) formulation

SYPRO Ruby protein gel stain is compatible with a variety of imaging platforms since it absorbs maximally in the ultraviolet (280 nm) and visible (470 nm) regions of the spectrum. Dye localization is achieved by noncovalent, electrostatic and hydrophobic binding to proteins, with signal being detected at 610 nm. Since proteins are not covalently modified by the dye, compatibility with downstream proteomics techniques such as matrix-assisted laser desorption/ionisation-time of flight mass spectrometry is assured. The principal limitation of the original formulation of SYPRO Ruby protein gel stain, is that it was only compatible with a limited number of gel fixation procedures. Too aggressive a fixation protocol led to diminished signal intensity and poor detection sensitivity. This is particularly apparent when post-staining gels subjected to labeling with other fluorophores such as Schiff's base staining of glycoproteins with fluorescent hydrazides. Consequently, we have developed an improved formulation of SYPRO Ruby protein gel stain that is fully compatible with commonly implemented protein fixation procedures and is suitable for post-staining gels after detection of glycoproteins using the green fluorescent Pro-Q Emerald 300 glycoprotein stain or detection of beta-glucuronidase using the green fluorescent ELF 97 beta-D-glucuronide. The new stain formulation is brighter, making it easier to manually excise spots for peptide mass profiling. An additional benefit of the improved formulation is that it permits staining of proteins in isoelectric focusing gels, without the requirement for caustic acids.

Simultaneous red/green dual fluorescence detection on electroblots using BODIPY TR-X succinimidyl ester and ELF 39 phosphate

A two-color fluorescence detection method is described based upon covalently coupling the succinimidyl ester of BODIPY TR-X dye to proteins immobilized on polyvinylidene difluoride membranes, followed by detection of target proteins using the fluorogenic, precipitating substrate ELF 39-phosphate in combination with alkaline phosphatase conjugated reporter molecules. This results in all proteins in the profile being visualized as fluorescent red signal while those detected specifically with the alkaline phosphatase conjugate appear as fluorescent green signal. The dichromatic detection system is broadly compatible with ultraviolet epi- or trans-illuminators combined with photographic or charge-coupled device cameras, and xenon-arc sources equipped with appropriate excitation/emission filters. The dichromatic method permits detection of low nanogram amounts of protein and allows for unambiguous identification of target proteins relative to the entire protein profile on a single electroblot, obviating the need to run replicate gels that would otherwise require visualization of total proteins by silver staining and subsequent alignment with chemiluminescent or colorimetric signals generated on electroblots. Combining the detection approach with an Alexa Fluor 350 dye conjugated monoclonal antibody permits simultaneous fluorescence detection of two antigens and the total protein profile on the same electroblot.

A whole new way of looking at things: the use of Dark Reader technology to detect fluorophors

The Dark Reader optical system (Clare Chemical Research, Denver, CO, USA) uses relatively low intensity broad-band visible blue light in combination with broad-band optical filters to detect fluorescence with a level of sensitivity that often surpasses that of UV transilluminators and can rival that of laser-based scanners. Applications of DR (Clare Chemical Research) devices include the detection of DNA and SYBR-stained protein samples following, and also during, electrophoresis. Unlike laser-based imaging systems, the fluorescence is directly visible to the user as well as being fully compatible with charge-coupled device (CCD) and Polaroid camera-based detection and imaging. Additionally, the DR optical system functions well in multicolor fluorophor environments. Because the Dark Reader does not emit any UV light, the extent of DNA damage incurred when visualizing DNA samples is drastically reduced compared to the damage produced by a UV device and this can have a significant benefit on downstream cloning protocols. Furthermore, dye photobleaching is minimal, extending the length of time that a fluorescent sample is visible. The inherent flexibility of the DR optical system allows many different configurations of the Dark Reader to be constructed such as transilluminators, hand lamps and integrated transilluminator-electrophoresis units.

Green/red dual fluorescence detection of total protein and alkaline phosphate-conjugated probes on blotting membranes

A two-color fluorescence detection method is described based upon covalently coupling the succinimidyl ester of BODIPY FL-X to proteins immobilized on poly(vinylidene difluoride) (PVDF) membranes, followed by detection of target proteins using the fluorogenic substrate 9H-(1,3-dichloro-9,9-dimethylacridin-2-one-7-yl(DDAO)-phosphate in combination with alkaline-phosphatase-conjugated reporter molecules. This results in all proteins in the profile being visualized as green signal while those detected specifically with the alkaline-phosphatase conjugate appear as red signal. The dichromatic detection system is broadly compatible with a wide range of analytical imaging devices including UV epi- or transilluminators combined with photographic or charge-coupled device (CCD) cameras, xenon-arc sources equipped with appropriate excitation/emission filters, and dual laser gel scanners outfitted with a 473 nm second-harmonic generation or 488 nm argon-ion laser as well as a 633 nm helium-neon or 635 nm diode laser. The dichromatic detection method permits detection of low nanogram amounts of protein and allows for unambiguous identification of target proteins relative to the entire protein profile on a single electroblot, obviating the need to run replicate gels that would otherwise require visualization of total proteins by silver staining and subsequent alignment with chemiluminescent or colorimetric signals generated on electroblots.