New Approaches for Absolute Quantification of Stable-Isotope-Labeled Peptide Standards for Targeted Proteomics Based on a UV Active Tag.

Targeted proteomics depends on the availability of stable isotope labeled (SIL) peptide standards, which for absolute protein quantification need to be absolutely quantified. In the present study, three new approaches for absolute quantification of SIL peptides are developed. All approaches rely on a quantification tag (Qtag) with a specific UV absorption. The Qtag is attached to the peptide during synthesis and is removed by tryptic digestion under standard proteomics workflow conditions. While one quantification method (method A) is designed to allow the fast and economic production of absolutely quantified SIL peptides, two other methods (methods B and C) are developed to enable the straightforward re-quantification of SIL peptides after reconstitution to control and monitor known problems related to peptide solubility, precipitation, and adhesion to vials. All methods yield consistent results when compared to each other and when compared to quantification by amino acid analysis. The precise quantitation methods are used to characterize the in vivo specificity of the H3 specific histone methyltransferase EZH2.

Detection of Histone Modification Dynamics during the Cell Cycle by MS-Based Proteomics.

DNA replication and subsequent deposition of nucleosomes is critical for the maintenance of the genome and epigenetic inheritance. Experiments using human tissue culture cells harvested at defined stages of the cell cycle can help to elucidate the mechanism of histone deposition and chromatin assembly in detail. Here, we describe a pulsed-SILAC approach to distinguish newly synthesized and deposited histones during S-phase of the cell cycle from parental "old" histones incorporated in previous replications and to decipher posttranslational histone modifications (PTMs).

Analysis of Histone Modifications by Mass Spectrometry.

Histone N termini undergo diverse post-translational modifications that significantly extend the information potential of the genetic code. Moreover, these modifications mark specific chromatin regions, modulating epigenetic control, lineage commitment, and overall function of chromosomes. It is widely accepted that histone modifications affect chromatin function, but the exact mechanisms by which modifications on histone tails and specific combinations of modifications are generated, and how they cross-talk with one another, are still enigmatic. Mass spectrometry is the gold-standard method for analyzing histone modifications, as it allows the quantification of modifications and combinations. This unit describes how high-resolution mass spectrometry can be used to study histone post-translational modifications. © 2018 by John Wiley & Sons, Inc.

Data on the kinetics of in vitro assembled chromatin.

Here, we use LC-MS/MS and SWATH-MS to describe the kinetics of in vitro assembled chromatin supported by an embryo extract prepared from preblastoderm Drosophila melanogaster embryos (DREX). This system allows easy manipulation of distinct aspects of chromatin assembly such as post-translational histone modifications, the levels of histone chaperones and the concentration of distinct DNA binding factors. In total, 480 proteins have been quantified as chromatin enriched factors and their binding kinetics have been monitored in the time course of 15 min, 1 h and 4 h of chromatin assembly. The data accompanying the manuscript on this approach, Völker-Albert et al., 2016 "A quantitative proteomic analysis of in vitro assembled chromatin" [1], has been deposited to the ProteomeXchange Consortium (http://www.proteomexchange.org) via the PRIDE partner repository with the dataset identifier submission number PRIDE: PXD002537 and PRIDE: PXD003445.

A Quantitative Proteomic Analysis of In Vitro Assembled Chromatin.

The structure of chromatin is critical for many aspects of cellular physiology and is considered to be the primary medium to store epigenetic information. It is defined by the histone molecules that constitute the nucleosome, the positioning of the nucleosomes along the DNA and the non-histone proteins that associate with it. These factors help to establish and maintain a largely DNA sequence-independent but surprisingly stable structure. Chromatin is extensively disassembled and reassembled during DNA replication, repair, recombination or transcription in order to allow the necessary factors to gain access to their substrate. Despite such constant interference with chromatin structure, the epigenetic information is generally well maintained. Surprisingly, the mechanisms that coordinate chromatin assembly and ensure proper assembly are not particularly well understood. Here, we use label free quantitative mass spectrometry to describe the kinetics of in vitro assembled chromatin supported by an embryo extract prepared from preblastoderm Drosophila melanogaster embryos. The use of a data independent acquisition method for proteome wide quantitation allows a time resolved comparison of in vitro chromatin assembly. A comparison of our in vitro data with proteomic studies of replicative chromatin assembly in vivo reveals an extensive overlap showing that the in vitro system can be used for investigating the kinetics of chromatin assembly in a proteome-wide manner.