An Approach for Triplex-Isobaric Peptide Termini Labeling (Triplex-IPTL)

Isobaric peptide termini labeling (IPTL) is based on labeling of both peptide termini with complementary isotopic labels resulting in isobaric peptides. MS/MS analysis after IPTL derivatization produces peptide-specific fragment ions which are distributed throughout the MS/MS spectrum. Thus, several quantification points can be obtained per peptide. In this report, we present triplex-IPTL, a chemical labeling strategy for IPTL allowing the simultaneous quantification of three states within one MS run. For this purpose, dimethylation of the N-terminal amino group followed by dimethylation of lysines was used with different stable isotopes of formaldehyde and cyanoborohydride. Upon LC-MS/MS analysis, the combined samples revealed three corresponding isotopic fragment ion series reflecting quantitatively the peptide ratios. To support this multiplexing labeling strategy, we have further developed the data analysis tool IsobariQ and included multidimensional VSN normalization, statistical inference, and graphical visualization of triplex-IPTL data and clustering of protein profiling patterns. The power of the triplex-IPTL approach in combination with IsobariQ was demonstrated through temporal profiling of HeLa cells incubated with the kinesin Eg5 inhibitor S-Trityl-l-cysteine (STLC). As a result, clusters of quantified proteins were found by their ratio profiles which corresponded well to their gene ontology association in mitotic arrest and cell death, respectively

An Approach for Triplex-Isobaric Peptide Termini Labeling (Triplex-IPTL)

Isobaric peptide termini labeling (IPTL) is based on labeling of both peptide termini with complementary isotopic labels resulting in isobaric peptides. MS/MS analysis after IPTL derivatization produces peptide-specific fragment ions which are distributed throughout the MS/MS spectrum. Thus, several quantification points can be obtained per peptide. In this report, we present triplex-IPTL, a chemical labeling strategy for IPTL allowing the simultaneous quantification of three states within one MS run. For this purpose, dimethylation of the N-terminal amino group followed by dimethylation of lysines was used with different stable isotopes of formaldehyde and cyanoborohydride. Upon LC-MS/MS analysis, the combined samples revealed three corresponding isotopic fragment ion series reflecting quantitatively the peptide ratios. To support this multiplexing labeling strategy, we have further developed the data analysis tool IsobariQ and included multidimensional VSN normalization, statistical inference, and graphical visualization of triplex-IPTL data and clustering of protein profiling patterns. The power of the triplex-IPTL approach in combination with IsobariQ was demonstrated through temporal profiling of HeLa cells incubated with the kinesin Eg5 inhibitor S-Trityl-l-cysteine (STLC). As a result, clusters of quantified proteins were found by their ratio profiles which corresponded well to their gene ontology association in mitotic arrest and cell death, respectively

Proximity Labeling Reveals Molecular Determinants of FGFR4 Endosomal Transport

The fibroblast growth factor receptors (FGFRs) are important oncogenes promoting tumor progression in many types of cancer, such as breast, bladder, and lung cancer as well as multiple myeloma and rhabdomyosarcoma. However, little is known about how these receptors are internalized and down-regulated in cells. We have here applied proximity biotin labeling to identify proteins involved in FGFR4 signaling and trafficking. For this purpose we fused a mutated biotin ligase, BirA*, to the C-terminal tail of FGFR4 (FGFR4-BirA*) and the fusion protein was stably expressed in U2OS cells. Upon addition of biotin to these cells, proteins in proximity to the FGFR4-BirA* fusion protein became biotinylated and could be isolated and identified by quantitative mass spectrometry. We identified in total 291 proteins, including 80 proteins that were enriched in samples where the receptor was activated by the ligand (FGF1), among them several proteins previously found to be involved in FGFR signaling (e.g., FRS2, PLCγ, RSK2 and NCK2). Interestingly, many of the identified proteins were implicated in endosomal transport, and by precise annotation we were able to trace the intracellular pathways of activated FGFR4. Validating the data by confocal and three-dimensional structured illumination microscopy analysis, we concluded that FGFR4 uses clathrin-mediated endocytosis for internalization and is further sorted from early endosomes to the recycling compartment and the trans-Golgi network. Depletion of cells for clathrin heavy chain led to accumulation of FGFR4 at the cell surface and increased levels of active FGFR4 and PLCγ, while AKT and ERK signaling was diminished, demonstrating that functional clathrin-mediated endocytosis is required for proper FGFR4 signaling. Thus, this study reveals proteins and pathways involved in FGFR4 transport and signaling that provide possible targets and opportunities for therapeutic intervention in FGFR4 aberrant cancer

Proximity Labeling Reveals Molecular Determinants of FGFR4 Endosomal Transport

The fibroblast growth factor receptors (FGFRs) are important oncogenes promoting tumor progression in many types of cancer, such as breast, bladder, and lung cancer as well as multiple myeloma and rhabdomyosarcoma. However, little is known about how these receptors are internalized and down-regulated in cells. We have here applied proximity biotin labeling to identify proteins involved in FGFR4 signaling and trafficking. For this purpose we fused a mutated biotin ligase, BirA*, to the C-terminal tail of FGFR4 (FGFR4-BirA*) and the fusion protein was stably expressed in U2OS cells. Upon addition of biotin to these cells, proteins in proximity to the FGFR4-BirA* fusion protein became biotinylated and could be isolated and identified by quantitative mass spectrometry. We identified in total 291 proteins, including 80 proteins that were enriched in samples where the receptor was activated by the ligand (FGF1), among them several proteins previously found to be involved in FGFR signaling (e.g., FRS2, PLCγ, RSK2 and NCK2). Interestingly, many of the identified proteins were implicated in endosomal transport, and by precise annotation we were able to trace the intracellular pathways of activated FGFR4. Validating the data by confocal and three-dimensional structured illumination microscopy analysis, we concluded that FGFR4 uses clathrin-mediated endocytosis for internalization and is further sorted from early endosomes to the recycling compartment and the trans-Golgi network. Depletion of cells for clathrin heavy chain led to accumulation of FGFR4 at the cell surface and increased levels of active FGFR4 and PLCγ, while AKT and ERK signaling was diminished, demonstrating that functional clathrin-mediated endocytosis is required for proper FGFR4 signaling. Thus, this study reveals proteins and pathways involved in FGFR4 transport and signaling that provide possible targets and opportunities for therapeutic intervention in FGFR4 aberrant cancer