| AMPK phosphorylated the catalytic domain of Raf 1 , expressed in Escherichia coli as a soluble GST fusion protein , to generate a single tryptic [ 32P ] phosphopeptide containing exclusively phospho Ser 621 . ^^^ AMPK also phosphorylated full length , kinase defective Raf 1 ( K375M ) to generate two [ 32P ] phosphopeptides , one co migrating with synthetic tryptic peptide containing phospho Ser 621 and the other with phospho Ser259 . . ^^^ |