Identification of Ser487as a phosphorylation site in Na
1.4 channel. Using previously reported methods , a purified fraction of sodium channels was prepared from control and CIM muscles and the excised band corresponding to the NaV 1.4 α subunit was excised, trypsinized, and analyzed by tandem mass spectrometry. This analysis indicated that the serine at 487 was partially phosphorylated, as we detected both the phosphorylated and unmodified forms in the samples. The MS/MS spectra of both forms were shown. The main product ions (b and y ions) were indicated. It is clear that a number of b ions ( b12 and b14) from the two peptides differ in a mass of 80 Da, the mass shift due to phosphorylation. In addition, neutral loss of phosphorus group was also observed for the modified form. The spectra shown are from a control sample. Although both control and CIM spectra were partially phosphorylated at the Ser487 site, the exact percentage was difficult to determine due to the small amount of sodium channel analyzed.