Updated publication reference for PubMed record(s): 31695196. Mass spectrometry-based proteomics was applied to unravel Erk signaling in mouse embryonic stem cells (ESCs). A previously described engineered ESCs system for Erk induction via a drug-inducible cRAF-ErT2 fusion (Hamilton, W. B. & Brickman, J. M., Cell Rep 2014) was used in two different setups measuring the phosphoproteome in an 11-plex tandem mass tag (TMT)-based approach as described below. Fgf4 stimulation and MEK inhibition: ESCs were stimulated with Fgf4 (40 nM, recombinant, mouse, 5 minutes) following pretreatment (30 minutes) with either DMSO (1:10000) or MEK inhibition (MEKi: 1 microM PD0325901): The experimental treatment conditions and TMT11-plex labeling are specified below: 126: DMSO, unstimulated control replicate 1 127N: DMSO, unstimulated control replicate 2 127C: DMSO, unstimulated control replicate 3 128N: DMSO + Fgf4 replicate 1 128C: DMSO + Fgf4 replicate 2 129N: DMSO + Fgf4 replicate 3 129C: MEKi + Fgf4 replicate 1 130N: MEKi + Fgf4 replicate 2 130C: MEKi + Fgf4 replicate 3 131N: MEKi replicate 1 131C: MEKi replicate 2 Analog sensitive Erk2 setup: In the ESC system the ERK2-/- ESCs (Hamilton et al., PloS ONE, 2013) were engineered to stably express an inducible cRAF-ErT2 fusion protein expressed from the same transgene as a FLAG-tagged analogue sensitive Erk2Q103A. Erk1 was subsequently removed by CRISPR-Cas9 mutagenesis using sgRNAs flanking exon 2. Cells were pre-treated for 30 minutes with either the ATP-mimetic compound 1-NM-PP1 (2 microM) (Endo et al., Biochem Biophys Res Commun 2006), the Mek1 inhibitor PD0325901 (1 microM), or DMSO (0.01%), followed by induction or not of the cRAF-ErT2 fusion by (Z)-4-Hydroxytamoxifen (4OHT) (250 nM) for 2 hours as indicated. The experimental treatment conditions and TMT11-plex labeling are specified below: 126: DMSO pretreatment, non-induced control, replicate 1 127N: DMSO pretreatment, non-induced control, replicate 2 127C: DMSO pretreatment, non-induced control, replicate 3 128N: DMSO pretreatment, 4OHT-induction, replicate 1 128C: DMSO pretreatment, 4OHT-induction, replicate 2 129N: DMSO pretreatment, 4OHT-induction, replicate 3 129C: PP1 pretreatment, 4OHT-induction, replicate 1 130N: PP1 pretreatment, 4OHT-induction, replicate 2 130C: PP1 pretreatment, 4OHT-induction, replicate 3 131N: MEKi pretreatment, 4OHT-induction, replicate 1 131C: MEKi pretreatment, 4OHT-induction, replicate 2