Ioanna Ntai

The emerging process of Top Down mass spectrometry for protein analysis: biomarkers, protein-therapeutics, and achieving high throughput

Top Down mass spectrometry (MS) has emerged as an alternative to common Bottom Up strategies for protein analysis. In the Top Down approach, intact proteins are fragmented directly in the mass spectrometer to achieve both protein identification and characterization, even capturing information on combinatorial post-translational modifications. Just in the past two years, Top Down MS has seen incremental advances in instrumentation and dedicated software, and has also experienced a major boost from refined separations of whole proteins in complex mixtures that have both high recovery and reproducibility. Combined with steadily advancing commercial MS instrumentation and data processing, a high-throughput workflow covering intact proteins and polypeptides up to 70 kDa is directly visible in the near future.

A Robust Two-Dimensional Separation for Top-Down Tandem Mass Spectrometry of the Low-Mass Proteome

For fractionation of intact proteins by molecular weight (MW), a sharply improved two-dimensional (2D) separation is presented to drive reproducible and robust fractionation before top-down mass spectrometry of complex mixtures. The “GELFrEE” (i.e., gel-eluted liquid fraction entrapment electrophoresis) approach is implemented by use of Tris-glycine and Tris-tricine gel systems applied to human cytosolic and nuclear extracts from HeLa S3 cells, to achieve a MW-based fractionation of proteins from 5 to >100 kDa in 1 h. For top-down tandem mass spectroscopy (MS/MS) of the low-mass proteome (5–25 kDa), between 5 and 8 gel-elution (GE) fractions are sampled by nanocapillary-LC-MS/MS with 12 or 14.5 tesla Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers. Single injections give about 40 detectable proteins, about half of which yield automated ProSight identifications. Reproducibility metrics of the system are presented, along with comparative analysis of protein targets in mitotic versus asynchronous cells. We forward this basic 2D approach to facilitate wider implementation of top-down mass spectrometry and a variety of other protein separation and/or characterization approaches.

Cell-free protein synthesis from a release factor 1 deficient Escherichia coli activates efficient and multiple site-specific nonstandard amino acid incorporation.

Site-specific incorporation of nonstandard amino acids (NSAAs) into proteins enables the creation of biopolymers, proteins, and enzymes with new chemical properties, new structures, and new functions. To achieve this, amber (TAG codon) suppression has been widely applied. However, the suppression efficiency is limited due to the competition with translation termination by release factor 1 (RF1), which leads to truncated products. Recently, we constructed a genomically recoded Escherichia coli strain lacking RF1 where 13 occurrences of the amber stop codon have been reassigned to the synonymous TAA codon (rEc.E13.ΔprfA). Here, we assessed and characterized cell-free protein synthesis (CFPS) in crude S30 cell lysates derived from this strain. We observed the synthesis of 190 ± 20 μg/mL of modified soluble superfolder green fluorescent protein (sfGFP) containing a single p-propargyloxy-l-phenylalanine (pPaF) or p-acetyl-l-phenylalanine. As compared to the parent rEc.E13 strain with RF1, this results in a modified sfGFP synthesis improvement of more than 250%. Beyond introducing a single NSAA, we further demonstrated benefits of CFPS from the RF1-deficient strains for incorporating pPaF at two- and five-sites per sfGFP protein. Finally, we compared our crude S30 extract system to the PURE translation system lacking RF1. We observed that our S30 extract based approach is more cost-effective and high yielding than the PURE translation system lacking RF1, ∼1000 times on a milligram protein produced/

Evolution of translation machinery in recoded bacteria enables multi-site incorporation of nonstandard amino acids

basis. Looking forward, using RF1-deficient strains for extract-based CFPS will aid in the synthesis of proteins and biopolymers with site-specifically incorporated NSAAs.

The first pilot project of the consortium for top-down proteomics: a status report.

Expansion of the genetic code with nonstandard amino acids (nsAAs) has enabled biosynthesis of proteins with diverse new chemistries. However, this technology has been largely restricted to proteins containing a single or few nsAA instances. Here we describe an in vivo evolution approach in a genomically recoded Escherichia coli strain for the selection of orthogonal translation systems capable of multi-site nsAA incorporation. We evolved chromosomal aminoacyl-tRNA synthetases (aaRSs) with up to 25-fold increased protein production for p-acetyl-L-phenylalanine and p-azido-L-phenylalanine (pAzF). We also evolved aaRSs with tunable specificities for 14 nsAAs, including an enzyme that efficiently charges pAzF while excluding 237 other nsAAs. These variants enabled production of elastin-like-polypeptides with 30 nsAA residues at high yields (∼50 mg/L) and high accuracy of incorporation (>95%). This approach to aaRS evolution should accelerate and expand our ability to produce functionalized proteins and sequence-defined polymers with diverse chemistries.

Optimizing capillary electrophoresis for top-down proteomics of 30–80 kDa proteins

Pilot Project #1—the identification and characterization of human histone H4 proteoforms by top‐down MS—is the first project launched by the Consortium for Top‐Down Proteomics (CTDP) to refine and validate top‐down MS. Within the initial results from seven participating laboratories, all reported the probability‐based identification of human histone H4 (UniProt accession P62805) with expectation values ranging from 10−13 to 10−105. Regarding characterization, a total of 74 proteoforms were reported, with 21 done so unambiguously; one new PTM, K79ac, was identified. Inter‐laboratory comparison reveals aspects of the results that are consistent, such as the localization of individual PTMs and binary combinations, while other aspects are more variable, such as the accurate characterization of low‐abundance proteoforms harboring >2 PTMs. An open‐access tool and discussion of proteoform scoring are included, along with a description of general challenges that lie ahead including improved proteoform separations prior to mass spectrometric analysis, better instrumentation performance, and software development.

Applying label-free quantitation to top down proteomics.

The direct analysis of intact proteins via MS offers compelling advantages in comparison to alternative methods due to the direct and unambiguous identification and characterization of protein sequences it provides. The inability to efficiently analyze proteins in the “middle mass range,” defined here as proteins from 30 to 80 kDa, in a robust fashion has limited the adoption of these “top‐down” methods. Largely, a result of poor liquid chromatographic performance, the limitations in this mass range may be addressed by alternative separations that replace chromatography. Herein, the short migration times of CZE‐ESI‐MS/MS have been extended to size‐sorted whole proteins in complex mixtures from Pseudomonas aeruginosa PA01. An electrokinetically pumped nanospray interface, a coated capillary, and a stacking method for on‐column sample concentration were developed to achieve high‐loading capacity and separation resolution. We achieved full width at half maximum of 8–16 s for model proteins up to 29 kDa and identified 30 proteins in the mass range of 30–80 kDa from P. aeruginosa PA01 whole cell lysate. These results suggest that CZE‐ESI‐MS/MS is capable of identifying proteins in the middle mass range in top‐down proteomics.

tRNA-dependent peptide bond formation by the transferase PacB in biosynthesis of the pacidamycin group of pentapeptidyl nucleoside antibiotics

With the prospect of resolving whole protein molecules into their myriad proteoforms on a proteomic scale, the question of their quantitative analysis in discovery mode comes to the fore. Here, we demonstrate a robust pipeline for the identification and stringent scoring of abundance changes of whole protein forms <30 kDa in a complex system. The input is ∼100–400 μg of total protein for each biological replicate, and the outputs are graphical displays depicting statistical confidence metrics for each proteoform (i.e., a volcano plot and representations of the technical and biological variation). A key part of the pipeline is the hierarchical linear model that is tailored to the original design of the study. Here, we apply this new pipeline to measure the proteoform-level effects of deleting a histone deacetylase (rpd3) in S. cerevisiae. Over 100 proteoform changes were detected above a 5% false positive threshold in WT vs the Δrpd3 mutant, including the validating observation of hyperacetylation of histone H4 and both H2B isoforms. Ultimately, this approach to label-free top down proteomics in discovery mode is a critical technical advance for testing the hypothesis that whole proteoforms can link more tightly to complex phenotypes in cell and disease biology than do peptides created in shotgun proteomics.

Proteomics-Based Discovery of Koranimine, a Cyclic-Imine Natural Product

Pacidamycins are a family of uridyl tetra/pentapeptide antibiotics with antipseudomonal activities through inhibition of the translocase MraY in bacterial cell wall assembly. The biosynthetic gene cluster for pacidamycins has recently been identified through genome mining of the producer Streptomyces coeruleorubidus, and the highly dissociated nonribosomal peptide assembly line for the uridyl tetrapeptide scaffold of pacidamycin has been characterized. In this work a hypothetical protein PacB, conserved in known uridyl peptide antibiotics gene clusters, has been characterized by both genetic deletion and enzymatic analysis of the purified protein. PacB catalyzes the transfer of the alanyl residue from alanyl-tRNA to the N terminus of the tetrapeptide intermediate yielding a pentapeptide on the thio-templated nonribosomal peptide synthetase (NRPS) assembly line protein PacH. PacB thus represents a new group of tRNA-dependent peptide bond-forming enzymes in secondary metabolite biosynthesis in addition to the recently identified cyclodipeptide synthases. The characterization of PacB completes the assembly line reconstitution of pacidamycin pentapeptide antibiotic scaffolds, bridging the primary and secondary metabolic pathways by hijacking an aminoacyl-tRNA to the antibiotic biosynthetic pathway.

Improving cell-free protein synthesis through genome engineering of Escherichia coli lacking release factor 1.

Nonribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) are large enzymes responsible for the biosynthesis of medically and ecologically important secondary metabolites. In a previous report, we described a proteomics approach to screen for expressed NRPSs or PKSs from bacteria with or without sequenced genomes. Here we used this proteome mining approach to discover a novel natural product arising from rare adenylation (A) and reductase (Red) domains in its biosynthetic machinery. We also cloned the entire gene cluster and elucidated the biosynthesis of the new compound, which is produced by an unsequenced Bacillus sp. isolated from soil collected in Koran, Louisiana.