Figure 1.

Identification and expression analysis of a novel N-terminally truncated hNaa40p proteoform, hNaa40^S. (A) Mass spectrometric (MS) identification of an Nt-truncated Naa40p proteoform (hNaa40^S) raised upon translation initiation at a downstream translation initiation start site (dTIS) as demonstrated by the MS²-based identification of the Nt-acetylated peptide MDAVCAKVDAANR (orange) matching amino acid sequence positions 22 to 34 of full-length canonical hNaa40^L (Q86UY6, length: 237 AA). The MS² spectrum matches a doubly charged precursor mass of m/z 762.3546, corresponding to the modified peptide sequence Ac-M < Mox > DAVC < Cmm > AK < AcD3 > VDAANR, and with ‘Ac-’, ‘Cmm’ and ‘AcD3’ denoting an acetyl, carbamidomethyl and (trideutero)acetyl moiety [21]. Tryptic peptide identifications reported in ProteomicsDB ([22], accessed January 2021) are indicated in bold in the hNaa40^L protein sequence overall providing a sequence coverage of 80.17% and with the reported AAMDAVCAK peptide indicative of hNaa40^L expression (Figure S1) (B) Expression profiling of endogenous hNaa40p in different human cell lines by Western blot analysis indicates expression of hNaa40^L in all 3 cell lines and prominent expression of the short proteoform of hNaa40p (hNaa40^S) in the pro-myelocytic leukemia cell lines HL-60. A non-specific band (*) serves as loading control.