When a synthetic peptide fragment (VAVVRTPPKSPSSAK) which corresponds to amino acid residues 226-240 from brain microtubule-associated protein tau was used as a testing substrate, we found that protein kinase FA/GSK-3 alpha was almost inactive towards this substrate. In sharp contrast, when Ser-10 of this peptide was replaced by a phosphoserine, the phosphopeptide fragment (VAVVRTPPKS(p)PSSAK) became an excellent substrate for kinase FA/GSK-3 alpha. Sequential manual Edman degradation together with phosphoamino acid analysis and protein sequencing further revealed that Thr-6 of the peptide fragment which corresponds to an important abnormal phosphorylation site Thr-231 in Alzheimer's diseased brain tau was the only site that was greatly phosphorylated, demonstrating that a pre-phosphorylation becomes a prerequisite and is essential for promoting phosphorylation of Thr-231. Taken together, the results provide initial evidence that kinase FA/GSK-3 alpha mediates a synergistic phosphorylation control mechanism involved in the abnormal site phosphorylation of Alzheimer's diseased brain tau.