FIGURE 1.

Detection of C7ORF49 isoform 2 (MRI-2) in K562 cells. A, peptidomics workflow to identify nonannotated short ORFs. The K562 cellular peptidome was isolated, fractionated, and subjected to liquid chromatography-mass spectrometry. Nonannotated peptides were identified by SEQUEST search against a K562 RNA deep sequencing library and subsequent removal of annotated sequences. B, MS/MS spectrum of the unique C7ORF49/MRI-2 C-terminal tryptic peptide, with detected fragment ions marked in blue (b ions) and red (y ions). C, multiple sequence alignment (ClustalW2) of the three MRI isoforms reveals that isoforms 1 and 2 have identical N-terminal sequences of 46 amino acids (red) before a frameshift generates a unique C-terminal sequence for isoform 2 (blue for isoform 2, green for isoform 1). Isoform 3 is identical to isoform 1, but lacks the N-terminal 46 amino acids.