Figure 2. β- and γ-actin are N-terminally acetylated. Mass spectrum of the N-terminal peptide of β-actin.

The protonated molecular ion (MH⁺) of the identified peptide has a mass of 1835.8 Da and is modified by removal of Met1 (m = 1722.9 Da), acetylation of Asp1 (indicated as d, Δm = +42 Da) and due to sample processing by an artificial propionamidation of cysteine (indicated as c, Δm = +71 Da). Similar results were obtained with γ-actin (spectrum not shown).