Enzymatic phosphorylation of soy globulins by the protein kinase CK2. Determination of the phosphorylation sites of beta-conglycinin alpha subunit by mass spectrometry.

ABSTRACT

Beta-conglycinin alpha subunit has been phosphorylated using a cAMP-independent protein kinase (CK2) purified from the yeast Yarrowia lipolytica. CK2 is known to phosphorylate serines and threonines in the consensus sequence Ser/Thr-X-X-Asp/Glu. Only 0.5 to 1 mol P/mol alpha subunit was incorporated although seven consensus sequences are present. Phosphorylated beta-conglycinin alpha subunit (P-alpha) was digested by trypsin. The resulting peptides were analysed by RP-HPLC coupled to electrospray ionisation mass spectrometry (LC-ESMS). Two phosphopeptides were identified corresponding to 70-89 and 116-127 sequences with Ser 75 and Ser 117 phosphorylated respectively. Ser 75 is one of the predicted phosphorylation sites according to the consensus sequence criteria. Ser 117 is inside a very acidic peptide but does not belong to a previously described consensus sequence.