Figure 3

Protein turnover demonstrated in mass spectra for a peptide from [¹³ C]-labeled ATP synthase CF1 β-subunit. Arabidopsis plants were grown with ¹³C-carbon dioxide in the enclosure for three weeks from seed then transferred to ambient air for 0 h, 24 h, 48 h and 96 h before the leaves were harvested for total protein extraction. Unlabeled proteins were added as a 'spike' to the [¹³C]-labeled samples before gel electrophoresis, protein band isolation, in-gel trypsin digestion and LC-MS/MS analysis. The observed spectra were fitted with three β-binomial distributions: natural abundance (green); newly synthesized peptide (red); and old peptide (blue) distributions shown for each spectrum in the insets. Sample spectra of the tryptic peptide from ATP synthase CF1 β -subunit (FVQAGSEVSALLGR, C₆₃H₁₀₄N₁₈O₂₀) show the disappearance of the ¹³C-labeled peptide over time. This peptide was doubly charged with a monoisotopic m/z of 717.391. In addition to a shift in the fractional isotopic abundance newly synthesized peptide (red) with time, the distribution abundance ratios of the newly synthesized peptide (red) and old peptide populations (blue) increase with time.