Transglutaminase Cross‐Linking of the τ Protein

Abstract: Tissue transglutaminase (EC 2.3.2.13) is a calcium‐activated enzyme that cross‐links specific substrate proteins into insoluble, protease‐resistant, high molecular weight complexes. Because the neurofibrillary tangles in Alzheimer disease have similar biochemical characteristics, and because the microtubule‐associated protein τ is the predominant component of these structures, the substrate properties of τ with respect to transglutaminase were investigated. Bovine τ and recombinant human τ isoforms rapidly form high molecular weight, cross‐linked polymers on incubation with transglutaminase. Polyamine incorporation assays indicate that bovine τ is an excellent substrate of transglutaminase, with a Km of 10.4 ± 2.2 µM and a Vmax of 40.9 ± 4.5 nmol/mg of enzyme/min. Individual recombinant human τ isoforms are not equivalent with respect to transglutaminase, as the smallest isoform T3 (352 amino acids) is not as good a substrate as the larger isoforms T4 (383 amino acids) and T4L (441 amino acids). To determine which segments of the τ protein are susceptible to modification by transglutaminase, τ was labeled with [3H]putrescine by transglutaminase and proteolyzed with α‐chymotrypsin, and the breakdown products were analyzed. These experiments demonstrate that the enzyme modifies τ at only one or a few discrete sites, primarily in the carboxyl half of the molecule. Thus, the reaction is specific for only a small number of the many glutamine residues in τ. Furthermore, a τ deletion construct (T264) containing a portion of the microtubule‐binding domains, which is a substrate of transglutaminase, cannot be cross‐linked by the enzyme. This provides evidence that the cross‐linking reaction is specific, and requires that the substrates be appropriately associated for cross‐linking to occur.