Figure 4.

Identification of S13 and S14 phosphorylation using top-down MS. (A) Top-down proteomics comparing Kir2.1^WT and Kir2.1^S425A both with PKA stimulation. Mass spectra were deconvoluted using the maximum entropy algorithm.⁶⁹ The intensity of each peak was quantified using the sum peak algorithm and is represented as a percentage $\left(\frac{{\text{intensity}}_{\text{proteoform}}}{{\text{intensity}}_{\text{total}}}\times 100\right)$. (B,C) Fragmentation map and representative fragment ions demonstrating successful localization of N-terminal phosphorylation using ETD. Circles represent theoretical isotopic distribution. The gray region in the protein sequence indicates the MYC tag.