The common techniques to study protein-protein proximity in vivo are not well-adapted to the capabilities and the expertise of a standard proteomics laboratory, typically based on the use of mass spectrometry. With the aim of closing this gap, we hav
e developed PUB-MS (for Proximity Utilizing Biotinylation and Mass Spectrometry), an approach to monitor protein-protein proximity, based on biotinylation of a protein fused to a biotin-acceptor peptide (BAP) by a biotin-ligase, BirA, fused to its interaction partner. The biotinylation status of the BAP can be further detected by either Western analysis or mass spectrometry. The BAP sequence was redesigned for easy monitoring of the biotinylation status by LC-MS/MS. In several experimental models, we demonstrate that the biotinylation in vivo is specifically enhanced when the BAP- and BirA- fused proteins are in proximity to each other. The advantage of mass spectrometry is demonstrated by using BAPs with different sequences in a single experiment (allowing multiplex analysis) and by the use of stable isotopes. Finally, we show that our methodology can be also used to study a specific subfraction of a protein of interest that was in proximity with another protein at a predefined time before the analysis.
Protein footprinting is a new methodology that is based on probing, typically with the use of mass spectrometry, of reactivity of different aminoacid residues to a modifying reagent. Data thus obtained allow one to make inferences about protein confo
rmations and their intermolecular interactions. Most of the protein footprinting studies so far have been performed on individual proteins in vitro. We explore whether a similar approach is possible with the proteins inside of living cells, employing dimethylsulfate (DMS), a reagent widely used for the in vivo footprinting of nucleic acids. DMS can induce methylation of the lysine, histidine and glutamate residues on proteins. Using models of the histone H2B/H2AZ heterodimer assembled in vitro and from chromatin treated in vivo, we show that the methylation by deuterated DMS allows one to distinguish the accessibility of a particular residue in and out of the proteins environmental/structural context. The detection of changes in protein conformations or their interactions in vivo can provide a new approach to the identification of proteins involved in various intracellular pathways and help in the search for perspective drug targets and biomarkers of diseases.
We describe a modification of the TAP method for purification and analysis of multiprotein complexes, termed here DEF-TAP (for Differential Elution Fractionation after Tandem Affinity Purification). Its essential new feature is the use for last purif
ication step of 6XHis-Ni++ interaction, which is resistant to a variety of harsh washing conditions, including high ionic strength and presence of organic solvents. This allows us to use various fractionation schemes before the protease digestion, which is expected to improve the coverage of the analysed protein mixture and also to provide an additional insight into the structure of the purified macromolecular complex and the nature of protein-protein interactions involved. We illustrate our new approach by analysis of soluble nuclear complexes containing histone H4 purified from HeLa cells. In particular, we observed different fractionation patterns of HAT1 and RbAp46 proteins as compared to RbAp48 protein, all identified as interaction partners of H4 histone. In addition, we report all components of the licensing MCM2-7 complex and the apoptosis-related DAXX protein among the interaction partners of the soluble H4. Finally, we show that HAT1 requires N-terminal tail of H4 for its stable association with this histone.
