Fig. 2: Detection of a GMP-modified SARS-CoV-2 peptide using LC–MS/MS.
From: Mass spectrometric based detection of protein nucleotidylation in the RNA polymerase of SARS-CoV-2
a Extracted ion chromatograms at the indicated mass-to-charge ratio (m/z) (+/− 5 parts per million) show precursor peptide MS signals versus retention time for the unmodified SARS-CoV-2 nsp7 peptide 1–14 with a charge of +2. Signals are shown for samples that were incubated without GTP (no nucleotide), with GTP, or with a mixture of GTP, 15N-GTP and 13C-GTP. b MS signals at the m/z of 928.4078 for these same samples. This mass corresponds to the GMP-modified version of the same peptide with a charge state of +2. c The average mass spectrum across these peaks is shown from the m/z range of 927–938 in order to visualize the presence or absence of the isotopic mass envelops for the GMP-labeled peptide and the corresponding 15N- and 13C-labeled GMP peptides. For the unlabeled sample, average mass spectrum for retention times of ~45.68–45.78 is shown. d The top-scoring EThcD peptide spectrum match for same GMP-labeled peptide is shown with labeling of main fragment ions. Ion series fragmentation for this peptide is illustrated with the asterisk indicating the GMP-modification site. Red, blue and green coloring indicate c- fragment ions, z- and y- fragment ions, and precursor ions, respectively. The GMP-modified SARS-CoV-2 nsp7 peptide was observed in two separate experiments each with two technical replicates.
