Large-scale analyses of the proteome have revealed proteomic changes in response to disease, and these changes hold great promise for diagnostics and treatment of complex disease if proteomic analysis can be brought into the clinical laboratory. Successful and reliable large-scale proteomics requires sample preparation workflows that are reproducible, reliable and show little variability. To bring proteomics into the clinical laboratory, standardized procedures and workflows for sample prep and analysis are required to generate valid, actionable results on a time scale useful for the clinic.
The two most common sample types analyzed for clinical proteomics are body fluids and tissue biopsies. To process these kinds of samples, there are two initial steps: tissue solubilization, followed by proteolytic digestion. Solubilization of solid tissues is the most labor-intensive and produces the most variable results.
The introduction of pressure cycling technology (PCT) using Barocycler instrumentation has greatly improved both tissue solubilization and digestion consistency. The PCT-based sample preparation protocols generally utilize urea as a lysis buffer for protein denaturing and solubilization. Urea has several drawbacks including inhibiting trypsin activity and introducing unwanted modifications like carbamylation.
Lucas and colleagues analyzed whether replacing urea with SDC would produce similar tissue digestion profiles and improve the PCT method.
SDC allowed the use of higher temperatures compared to urea, and hence the first step (lysis, reduction, and alkylation) was performed at 56 °C. The second digestion step in the Barocycler was optimized, and the third step was eliminated. To further reduce digestion time, they capitalized on Rapid Trypsin/Lys-C. Rapid Trypsin/Lys-C maintains robust activity at 70 °C, and allowed Barocycler digestion to be performed in a single step, completing digestion in 30 cycles (approximately 30 min) rather than 105 minutes, streamlining the protocol.
The data presented an improved conventional tissue PCT approach in a Barocycler by replacing urea and proteolytic enzymes with SDC, N-propanol, and modified commercially available enzymes that have higher optimum temperatures.
Lucas, N. et al. (2019) Accelerated Barocycler Lysis and Extraction Sample Preparation for Clinical Proteomics by Mass Spectrometry. J of Proteome Res 18, 399–405.
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.
- Laskay, U. et al. (2013) Proteome Digestion Specificity Analysis for the Rational Design of Extended Bottom-up and middle-down proteomics experiments. J of Proteome Res. 12, 5558–69.
- Giansanti, P. et. al. (2016) Six alternative protease for mass spectrometry based proteomics beyond trypsin. Nat. Protocols 11, 993–6