DOI 10.1007/s00702-005-0313-5 J Neural Transm (2006) 113: 219–230 PP2B isolated from human brain preferentially dephosphorylates Ser-262 and Ser-396 of the Alzheimer disease abnormally hyperphosphorylated tau A. Rahman, I. Grundke-Iqbal, and K. Iqbal Department of Neurochemistry, New York State Institute for Basic Research in Developmental Disabilities, Staten Island, NY, USA Received December 6, 2004; accepted March 30, 2005 Published online June 15, 2005; # Springer-Verlag 2005 Summary. PP2B is one of the major serine= threonine phosphatases in the brain. We quantitated the dephosphorylation of various sites of Alzheimer disease abnormally hyper- phosphorylated tau by PP2B purified from six (three Alzheimer and three control) au- topsied human brains. The purified PP2B was essentially homogenous holoenzyme as determined by SDS-PAGE, Western blot analyses and biochemical characterization. Purified PP2B from all six brains efficient- ly dephosphorylated 32 P-tau with specific activities ranging from 684–1286 pmol 32 Pi=mg=min. Estimated by dot-blot anal- yses, the purified PP2B (on average from six brains) dephosphorylated Alzheimer tau at pS199, pT217, pS262, pS396 and pS422 by 38%, 32%, 63%, 78%, and 32%, respec- tively. Dephosphorylation of tau at pT181, pS202, pT205, pT212, pS214, and pS404 by PP2B was undetectable. The preferential dephosphorylation of Ser262 and Ser396 by PP2B suggests a possible involvement of this phosphatase in Alzheimer neurofibrillary degeneration. Keywords: Protein phosphatase-2B, cal- cineurin, tau protein, Alzheimer disease, neurofibrillary tangles, abnormally hyperphos- phorylated tau. Introduction Tau from Alzheimer disease (AD) brain is phosphorylated at multiple sites and in this form is assembled into paired helical filaments (PHF) of neurofibrillary tangles (Grundke-Iqbal et al., 1986a, b). At least 30 serine=threonine phosphorylation sites have been identified in abnormally hyperphos- phorylated tau from AD brain (ADP-tau) (for review, see Gong et al., 2005). In AD brain, tau can be hyperphosphorylated by an increase in the activities of one or more serine=threonine kinases or reduced activity of a phosphoseryl=phosphothreonyl protein phosphatase(s). Identification of the protein kinases and phosphatases involved in the generation of abnormally hyperphosphorylated tau is crit- ical to understanding the pathogenesis of AD because neurofibrillary degeneration is not only a hallmark lesion of this disease but also the number of neurofibrillary tangles direct- ly correlates to the degree of dementia (see Tomlinson et al., 1970; Alafuzoff et al., 1987;