The spike protein of the SARS-CoV-2 virus is a very commonly researched target in COVID-19 vaccine and therapeutic studies because it is an integral part of host cell entry through interactions between the S1 subunit of the spike protein with the ACE2 protein on the target cell surface. Viral proteins important in host cell entry are typically highly glycosylated. Looking at the sequence of the SARS-CoV-2 virus, researchers predict that the spike protein is highly glycosylated. In a recent study, spike proteins of SARS-CoV-2 were analyzed using mass spec analysis to determine the N-glycosylation profile of the subunits that make up the spike protein.
Glycans assist in protein folding and help the virus avoid immune recognition by the host. Glycosylation can also have an impact on the antigenicity of the virus, as well as potential effects on vaccine safety and efficacy. Mass spectrometry is widely used for viral characterization studies of influenza viruses. Specifically, mass spec has been used to study influenza protein glycosylation, antigen quantification, and determination of vaccine potency.
In older people, low muscle mass is strongly associated with reduced functional capacity and an increased risk of disability. Myostatin is a negative regulator of muscle growth and has become an important target for pharmaceutical companies designing therapeutics to address age-associated muscle loss.
Anti-myostatin drugs increase muscle size and strength in preclinical studies. Fortetropin is a proteo-lipid complex made from fertilized egg yolk and shows anti-myostatin activity. When Fortetropin is provided as a supplement, lowered circulating myostatin levels are observed both in rodents and in young men. Fortetropin in combination with resistance exercise also lowers myostatin and increased lean body mass.
Sometimes, when using trypsin to study a protein sequence or protein modifications, sequence coverage just isn’t quite as complete as you’d like. Looking for a protease with novel cleavage specificity or a protease that functions well in a low pH environment? Promega has a protease for that.
ProAlanase is a new site-specific endoprotease that preferentially cleaves proteins on the C-terminal side of proline and alanine amino acids. The unique cleavage specificity of ProAlanase (also known as An-PEP or EndoPro; 1–3) can help to uncover parts of the proteome not previously accessible with proteases typically used in proteomic studies.
Protein:protein interactions (PPIs) play a key role in regulating cellular activities including DNA replication, transcription,translation, RNA splicing, protein secretion, cell cycle control and signal transduction. A comprehensive method is needed to identify the PPIs before the significance of the protein:protein interactions can be characterized. Affinity purification−mass spectrometry (AP−MS) has become the method of choice for discovering PPIs under native conditions. This method uses affinity purification of proteins under native conditions to preserve PPIs. Using this method, the protein complexes are captured by antibodies specific for the bait proteins or for tags that were introduced on the bait proteins and pulled down onto immobilized protein A/G beads. The complexes are further digested into peptides with trypsin. The protein interactors of the bait proteins are identified by quantification of the tryptic peptides via mass spectrometry.
The success of AP-MS depends on the efficiency of trypsin digestion and the recovery of the tryptic peptides for MS analysis. Several different protocols have been used for trypsin digestion of protein complexes in AP-MS studies, but no systematic studies have been conducted on the impact of trypsin digestion conditions on the identification of PPIs. A recent publication used NFB/RelA and BRD4 as bait proteins and five different trypsin digestion conditions (two using “on beads” and three using “elution” digestion protocols). Although the performance of the trypsin digestion protocols changed slightly depending on the different bait proteins, antibodies and cell lines used, the authors of the paper found that elution digestion methods consistently outperformed on-beads digestion methods.
While many proteases are used in bottom-up mass spectrometric (MS) analysis, trypsin (4,5) is the de facto protease of choice for most applications. There are several reasons for this: Trypsin is highly efficient, active and specific. Tryptic peptides produced after proteolysis are ideally suited, in terms of both size (350–1,600 Daltons) and charge (+2 to +4), for MS analysis. One significant drawback to trypsin digestion is the long sample preparation times, which typically range from 4 hours to overnight for most protocols. Achieving efficient digestion usually requires that protein substrates first be unfolded either with surfactants or denaturants such as urea or guanidine. These chemical additives can have negative effects, including protein modification, inhibition of trypsin or incompatibility with downstream LC-MS/MS. Accordingly, additional steps are typically required to remove these compounds prior to analysis.
To shorten the time required to prepare samples for LC-MS/MS analysis, we have developed a specialized trypsin preparation that supports rapid and efficient digestion at temperatures as high as 70°C. There are several benefits to this approach. First, proteolytic reaction times are dramatically shortened. Second, because no chemical denaturants have been added, off -line sample cleanup is not necessary, leading to shorter preparation times and diminished sample losses.
The Rapid Digestion trypsin protocols are highly flexible. They can accommodate a variety of additives including reducing and alkylating agents. There are no restrictions on sample volume or substrate concentrations with these kits. Furthermore, the protocol is simple to follow and requires no laboratory equipment beyond a heat block. Digestion is achieved completely using an in-solution approach, and since the enzyme is not immobilized on beads, the protocol does not have strict requirements for rapid shaking and off -line filtering to remove beads.
In addition to the benefits of this flexibility, we also developed a Rapid Digestion–Trypsin/Lys-C mixture. Like the Trypsin/Lys-C Mix previously developed to prepare maximally efficiently proteolytic digests, particularly for complex mixtures, Rapid Digestion–Trypsin/Lys C is ideally suited for studies that require improved reproducibility across samples.
As more and more protein-based therapeutics enter research pipelines, more efficient protocols are needed for characterization of protein structure and function, as well as means of quantitation. One main step in this pipeline, proteolysis of these proteins into peptides, presents a bottleneck and can require optimization of multiple steps including reduction, alkylation and digestion time.
We have developed a new trypsin reagent, Rapid Digestion–Trypsin, that streamlines the protein sample preparation process, reducing the time to achieve proteolysis to about 1 hour, a remarkable improvement over existing overnight sample preparation times.
How Does it Work?
With this new trypsin product, proteolysis is performed at 70°C, incorporating both denaturation and rapid digestion. The protocol can be used with multiple protein types, including pure proteins and complex mixtures, and is compatible with digestion under native, reduced or nonreduced conditions.
Bottom-up proteomics focuses on the analysis of protein mixtures after enzymatic digestion of the proteins into peptides. The resulting complex mixture of peptides is analyzed by reverse-phase liquid chromatography (RP-LC) coupled to tandem mass spectrometry (MS/MS). Identification of peptides and subsequently proteins is completed by matching peptide fragment ion spectra to theoretical spectra generated from protein databases.
Trypsin has become the gold standard for protein digestion to peptides for shotgun proteomics. Trypsin is a serine protease. It cleaves proteins into peptides with an average size of 700-1500 daltons, which is in the ideal range for MS (1). It is highly specific, cutting at the carboxyl side of arginine and lysine residues. The C-terminal arginine and lysine peptides are charged, making them detectable by MS. Trypsin is highly active and tolerant of many additives.
Even with these technical features, the use of trypsin in bottom-up proteomics may impose certain limits in the ability to grasp the full proteome, Tightly-folded proteins can resist trypsin digestion. Post-translational modifications (PTMs) present a different challenge for trypsin because glycans often limit trypsin access to cleavage sites, and acetylation makes lysine and arginine residues resistant to trypsin digestion.
To overcome these problems, the proteomics community has begun to explore alternative proteases to complement trypsin. However, protocols, as well as expected results generated when using these alternative proteases have not been systematically documented.
In a recent reference (2), optimized protocols for six alternative proteases that have already shown promise in their applicability in proteomics, namely chymotrypsin, Lys-C, Lys-N, Asp-N, Glu-C and Arg-C have been created.
Data describe the appropriate MS data analysis methods and the anticipated results in the case of the analysis of a single protein (BSA) and a more complex cellular lysate (Escherichia coli). The digestion protocol presented here is convenient and robust and can be completed in approximately in 2 days.
Biomarkers in biological fluids in particular have the potential to inform regarding risk of disease or to allow early detection for more effective treatment. Plasma/serum is considered the universal source of biomarkers. This fluid is, indeed, easily collected, and the important point is that plasma collects proteins from each and every tissue, compared to other fluids such as urine or cerebrospinal fluid. Optimizing experimental conditions (i.e., use of trypsin for the digestion of target proteins) used to discover or monitor biomarkers in plasma is critical to successful detection of biomarkers.
In a recent publication by Proc et al., plasma denaturation/digestion protocols were compared using quanititation methods. In this reference 14 combinations of heat, solvent (acetonitrile, methanol, trifluoroethanol), chaotropic agents (guanidine hydrochloride, urea) and surfactants (sodium dodecyl sulfate (SDS)and sodium deoxycholate (DOC) with effectiveness in improving tryptic digestion. Digestion efficiency was monitored by quantitating the peptides from 45 moderate- to high-abundance plasma proteins using tandem mass spectrometry in multiple reaction mode with a mixture of stable isotope labeled analogues of these peptides as internal standards. In the results, Proc et al. noted that use of either DOC and SDS produced an increase in the overall yield of tryptic peptides. Since SDS is not compatible with mass spectrometry and DOC can be easily remove by acid precipitation, the overall recommendation was the use of DOC with a nine hour digestion procedure.
Protein phosphorylation is the most widespread type of post-translational modification. It affects every basic cellular process, including metabolism, growth, division, differentiation, motility, organelle trafficking, membrane transport, muscle contraction, immunity, learning and memory (1,2). Protein kinases catalyse the transfer of the phosphate from ATP to specific amino acids in proteins. In eukaryotes, these are usually Ser, Thr and Tyr residues. Due to the development of specific phosphopeptide enrichment techniques and highly sensitive MS instruments, phosphoproteomics has enabled researchers to gain a comprehensive view on the dynamics of protein phosphorylation and phosphorylation based signaling networks.
Due to its high cleavage specificity, trypsin is the commonly used proteolytic enzyme in MS-based proteomics, cleaving peptides carboxyterminal of the amino acids lysine and arginine. However, various factors such as the tertiary structure of a protein, adjacent basic amino acids or negatively charged residues close to cleavage sites as well as PTMs are known to impair proteolysis.
To gain closer insights into the impact of phosphorylation on tryptic digestion, a recent publication(3) systematically characterized the digestion efficiency of model peptide sequences that are known to be prone to incomplete digestion.
The results indicated that increasing trypsin concentrations up to a trypsin to peptide ratio of 1:10 led to a significant gain (1) in the overall number of phosphorylation sites (up to 9%) and in the intensities of individual phosphopeptides, thereby improving the sensitivity of phosphopeptide quantification.
The effect of organic solvents (ACN, acetonitrile and TFE trifuorethanol was also evaluated). Positive results were noted with TFE when determining the digestion of individual peptides. However TFE interfered with TiO2 phosphopeptide enrichment and therefore was not recommended for use with complex samples.
Efficient proteolysis is a major requirement for protein mass spectrometry analysis. Incomplete digestion has multiple ramifications including decreased number of identified proteins, compromised analytical reproducibility and protein quantitation, etc. Trypsin is one of the most robust proteases and is characterized by efficient proteolysis. Typical trypsin reactions do not digest proteins to completion, missing 15–30% of cleavage sites. Incomplete digestion affects protein identification, reproducibility of mass spectrometry analysis and accuracy of protein quantitation. Supplementing Trypsin with Lys-C compensates for the majority of missed cleavages. Continue reading “Enhanced Protein Mass Spectrometry Analysis with Trypsin/Lys-C Mix”