ABRF-sPRG 2018-2019: Development and Characterization of a Stable-Isotope Labeled Phosphopeptide Standard.

The mission of the ABRF Proteomics Standards Research Group (sPRG) is to identify and implement technical standards that reflect the ABRF's commitment to accuracy, clarity, and consistency in the field of proteomics. There is broad interest in quantifying protein phosphorylation alterations in cellular signaling pathways under different conditions. The transient nature and low abundance of many phosphorylation sites makes this analysis challenging. Here we report on the follow up of the two-year sPRG study designed to target various issues encountered in phosphopeptide experiments. We have constructed a pool of heavy-labeled phosphopeptides that will enable core facilities to rapidly develop assays. Our pool contains over 150 phosphopeptides that have been previously observed in mass spectrometry data sets. The specific peptides have been chosen to cover as many known biologically interesting phosphosites as possible from seven different signaling pathways: AMPK, death and apoptosis, ErbB, insulin/IGF-1, mTOR, PI3K/AKT, and stress (p38/SAPK/JNK). We feel this pool will enable researchers to test the effectiveness of their enrichment workflows and to provide a benchmark for a cross lab study. This standard should be helpful in number of ways, including providing a complete workflow solution for phosphopeptide enrichment, as an internal enrichment and chromatography calibrant, and as a pre-built biological assay for a wide variety of signaling pathways. Previously, we mixed the standard into an activated HeLa tryptic digest and distributed the mixture to over 60 ABRF member and nonmember laboratories around the world. We asked participants to enrich phosphopeptides out of the HeLa background and report ratios of the heavy phosphopeptides to the endogenous levels. In the current study, we continue validation of the standard within various RG group/ABRF members' laboratories. The aim of this follow up study is to provide reagents, an optimized phosphopeptide enrichment protocol, instrument acquisition method parameters, and data analysis templates.