CROSS-LINKING SITES OF THE HUMAN TAU-PROTEIN, PROBED BY REACTIONS WITH HUMAN TRANSGLUTAMINASE

A portion of the neurofibrillary tangles of Alzheimar's disease has th e characteristics of cross-linked protein. Because the principal compo nent of these lesions is the microtubule-associated protein tau, and b ecause a major source of cross-linking activity within neurons is supp lied by tissue transglutaminase (TGase), it has been postulated that i sopeptide bond formation is a major posttranslational modification lea ding to the formation of insoluble neurofibrillary tangles. Here we ha ve mapped the sites on two isoforms of human tau protein (tau 23 and t au 40) capable of participating in human TGase-mediated isopeptide bon d formation. Using dansyl-labeled fluorescent probes, it was shown tha t eight Gin residues can function as amine acceptor residues, with two major sites being Gln(351) and Gln(424). In addition, 10 Lys residues were identified as amine donors, most of which are clustered adjacent to the microtubule-binding repeats of tau in regions known to be solv ent accessible in filamentous tau. The distribution of amine donors co rrelated closely with that of Arg residues, suggesting a link between neighboring positive charge and the TGase selectivity for donor sites in the protein substrate. Apart from revealing the sites that can be c ross-linked during the TGase-catalyzed assembly of tau filaments, the results suggest a topography for the tau monomers so assembled.