Posttranslational regulation of self-renewal capacity: insights from proteome and phosphoproteome analyses of stem cell leukemia.

We recently generated 2 phenotypically similar Hoxa9+Meis1 overexpressing acute myeloid leukemias that differ by their in vivo biologic behavior. The first leukemia, named FLA2, shows a high frequency of leukemia stem cells (LSCs; 1 in 1.4 cells), whereas the second, FLB1, is more typical with a frequency of LSCs in the range of 1 per several hundred cells. To gain insights into possible mechanisms that determine LSC self-renewal, we profiled and compared the abundance of nuclear and cytoplasmic proteins and phosphoproteins from these leukemias using quantitative proteomics. These analyses revealed differences in proteins associated with stem cell fate, including a hyperactive p38 MAP kinase in FLB1 and a differentially localized Polycomb group protein Ezh2, which is mostly nuclear in FLA2 and predominantly cytoplasmic in FLB1. Together, these newly documented proteomes and phosphoproteomes represent a unique resource with more than 440 differentially expressed proteins and 11 543 unique phosphopeptides, of which 80% are novel and 7% preferentially phosphorylated in the stem cell-enriched leukemia.

Discovery of protein acetylation patterns by deconvolution of peptide isomer mass spectra.

Protein post-translational modifications (PTMs) play important roles in the control of various biological processes including protein-protein interactions, epigenetics and cell cycle regulation. Mass spectrometry-based proteomics approaches enable comprehensive identification and quantitation of numerous types of PTMs. However, the analysis of PTMs is complicated by the presence of indistinguishable co-eluting isomeric peptides that result in composite spectra with overlapping features that prevent the identification of individual components. In this study, we present Iso-PeptidAce, a novel software tool that enables deconvolution of composite MS/MS spectra of isomeric peptides based on features associated with their characteristic fragment ion patterns. We benchmark Iso-PeptidAce using dilution series prepared from mixtures of known amounts of synthetic acetylated isomers. We also demonstrate its applicability to different biological problems such as the identification of site-specific acetylation patterns in histones bound to chromatin assembly factor-1 and profiling of histone acetylation in cells treated with different classes of HDAC inhibitors.

Enhanced sensitivity in proteomics experiments using FAIMS coupled with a hybrid linear ion trap/Orbitrap mass spectrometer.

We describe the use and application of high-field asymmetric waveform ion mobility spectrometry combined with nanoscale liquid chromatography mass spectrometry (nanoLC-FAIMS-MS) to improve the sensitivity and dynamic range of proteomics analyses on a hybrid LTQ-Orbitrap mass spectrometer. The ability of FAIMS to enrich multiply protonated peptides against background ions confers a marked advantage in proteomics analyses by decreasing the limits of detection to facilitate the identification of low-abundance peptide ions. These multiply charged ions are recorded into separate acquisition channels to enhance the overall population of detectable peptide ions from a single analysis. NanoLC-FAIMS-MS experiments performed on peptides spiked into complex proteins digests provided more than 10-fold improvement in limits of detection compared to conventional nanoelectrospray mass spectrometry. This enhancement of sensitivity is reflected by a 55% increase in the number of assigned MS/MS spectra contributing to an overall improvement in protein identification and sequence coverage. The application of FAIMS in label-free quantitative proteomics is demonstrated for the identification of differentially abundant proteins from human U937 monocytic cells exposed to phorbol ester.