Arg-10 conversion is prevented in arginase-deficient cells.

<p>Mass spectra of tryptic peptides (DAEGMSHIWQLR and AIQLELENLSSQAFR in (A) and (B), respectively) from <i>S</i>. <i>pombe</i> Mto1 protein (SPCC417.07c) isolated from (i, ii) <i>car2∆ arg1-230 lys3-37</i> or (iii) <i>car1∆ aru1∆ arg1-230 lys3-37</i> cells, grown in either (i) unlabeled arginine (Arg-0) and unlabeled lysine (Lys-0), or (ii, iii) ¹³C₆¹⁵N₄-arginine (Arg-10) and ¹³C₆¹⁵N₂-lysine (Lys-8), as indicated. In peptides isolated from <i>car2∆</i> cells grown in Arg-10, extensive conversion is observed, resulting in both higher-molecular-mass peaks and lower-molecular-mass “pre-peaks” (ii; see also Figs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0129548#pone.0129548.g001" target="_blank">1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0129548#pone.0129548.g002" target="_blank">2</a>). These are not observed in peptides from <i>car1∆ aru1∆</i> cells grown in Arg-10 (iii). Mass-to-charge (m/z) ratios of monoisotopic peaks for each growth condition (i.e., assuming no conversion of labeled arginine) and inferred peptide charge-states are indicated in magenta. Pre-peaks are indicated in blue.