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Labeling peptides with isobaric tags is a popular strategy in quantitative bottom-up proteomics. In this study, we labeled six breast tumor cell lysates (1.34 mg proteins per channel) using 10-plex tandem mass tag reagents and analyzed the samples on a Q Exactive HF Quadrupole-Orbitrap mass spectrometer. We identified a total of 8706 proteins and 28186 phosphopeptides, including 7394 proteins and 23739 phosphosites common to all channels. The majority of technical replicates correlated with a R2 ≥ 0.98, indicating minimum variability was introduced after labeling. Unsupervised hierarchical clustering of phosphopeptide datasets successfully classified the breast tumor samples into Her2 (epidermal growth factor receptor 2) positive and Her2 negative groups, whereas mRNA abundance did not. The tyrosine phosphorylation levels of receptor tyrosine kinases, phosphoinositide-3-kinase, protein kinase C delta and Src homology 2, among others, were significantly higher in the Her2 positive than the Her2 negative group. Despite ratio compression in MS2-based experiments, we demonstrated the ratios calculated using an MS2 method are highly correlated (R2 > 0.65) with ratios obtained using MS3-based quantitation (using a Thermo Orbitrap Fusion mass spectrometer) with reduced ratio suppression. Given the deep coverage of global and phosphoproteomes, our data show that MS2-based quantitation using TMT can be successfully used for large-scale multiplexed quantitative proteomics.

Analysis of phosphopeptides from complex mixtures derived from proteolytic digestion of biological samples is a challenging yet highly important task. Since phosphopeptides are usually present in small amounts, enrichment is often necessary prior to their characterization by mass spectrometry. In this study, a thin layer of titanium dioxide (TiO2) nanoparticles (NPs) was deposited onto the surface of capillary column by liquid phase deposition (LPD) technique and applied to selectively concentrate phosphopeptides from protein digest products. This is, to our knowledge, the first demonstration of using liquid phase deposition to construct in-tube solid phase microextraction devices for biological analysis. By coupling the device off-line or on-line with mass spectrometry analysis, experiments for systematic optimization of loading and washing conditions were carried out, and good trapping selectivity of TiO2 NP-deposited capillary columns towards phosphopeptides was demonstrated.

Determination of a protein’s N-terminal sequence can be important for the characterization of protein processing. To increase the confidence of protein N-terminal identification, chemical derivatization of the N-terminal amine group by (N-Succinimidyloxycarbonylmethyl)tris(2,4,6-trimethoxyphenyl)phosphonium bromide (TMPP) or dimethyl labeling followed by mass spectrometric analysis is commonly performed. Using this approach, proteins can be separated by SDS-PAGE, and the protein N-terminus of interest is labeled by TMPP or dimethyl in-gel before tryptic digestion and LC-MS analysis. The N-terminus of the protein can thus be easily identified because only the N-terminal tryptic peptide contains the labeling. Peptides with N-terminal derivatization produce a better fragmentation pattern during tandem mass spectrometric analysis, which significantly facilitates sequencing of these peptides.

Filopodia are cell surface protrusions that are essential for cell migration. This finger-like structure is supported by rigid tightly bundled actin filaments. The protein responsible for actin bundling in filopodia is fascin. However, the mechanism by which fascin functions in filopodial formation is not clear. Here we provide biochemical, cryo-electron tomographic, and x-ray crystal structural data demonstrating the unique structural characteristics of fascin. Systematic mutagenesis studies on 100 mutants of fascin indicate that there are two major actin-binding sites on fascin. Crystal structures of four fascin mutants reveal concerted conformational changes in fascin from inactive to active states in the process of actin bundling. Mutations in any one of the actin-binding sites impair the cellular function of fascin in filopodial formation. Altogether, our data reveal the molecular mechanism of fascin function in filopodial formation.