Figure 3.

Identification of S425 phosphorylation using top-down MS. (A) Intact mass spectra of Kir2.1^WT with and without 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 phosphorylation to S425 using online CAD. Circles represent theoretical isotopic distribution. Phosphorylation sites are reported based on a native sequence (i.e., without the MYC tag).