Hyperphosphorylated tau is an integral part of the neurofibrillary tangles that form within neuronal cell bodies, and
tau protein kinase II is reported to play a role in the pathogenesis of
Alzheimer's disease. Recently, we reported that
tau protein kinase II (cdk5/p20)-phosphorylated human tau inhibits microtubule assembly, and
tau protein kinase II (cdk5/p20) phosphorylation of microtubule-associated tau results in dissociation of phosphorylated tau from the microtubules and
tubulin depolymerization. In the studies reported here, a combination of mass spectrometric techniques was used to study the phosphorylation of human recombinant tau by recombinant
tau protein kinase II (cdk5/p20) in vitro. The extent of phosphorylation was determined by measuring the molecular mass of phosphorylated tau using mass spectrometry. Reaction of human recombinant tau with
tau protein kinase II (cdk5/p20) resulted in the formation of two major species containing either five or six
phosphate groups. The specific
amino acid residues phosphorylated were determined by analyzing tryptic
peptides by tandem mass spectrometry via either MALDI/TOF post-source decay or by electrospray tandem mass spectrometry. Based on these experiments, we conclude that
tau protein kinase II (cdk5/p20) can phosphorylate human tau at Thr(181), Thr(205), Thr(212), Thr(217), Ser(396) and Ser(404).