Specific processing of native and phosphorylated tau protein by proteases.

Protein tau is a group of developmentally regulated proteins with implications in the pathogenesis of Alzheimer's disease (AD). To study whether phosphorylation of tau by different protein kinases may affect subsequent reactivity to proteases, human recombinant tau-3 was phosphorylated with PKA or a double-stranded DNA-dependent protein kinase (referred to as DNA-PK), followed by incubation with thrombin or a double-stranded DNA (dsDNA)-stimulated protease. Quantitative degradation of tau was measured by the disappearance of the substrates or the appearance of products on SDS-PAGE. With thrombin, tau-3 phosphorylated by DNA-PK was degraded faster than the native protein which was processed at a faster rate than tau-3 phosphorylated by PKA. With the dsDNA-stimulated protease, however, tau-3 phosphorylated by PKA was processed faster than that phosphorylated by DNA-PK. Thrombin-mediated degradation of DNA-PK phosphorylated tau-3 gave a pattern different from that using the native or the PKA phosphorylated tau-3 as substrates. These results suggest that the rate and/or sequence of phosphorylation at specific sites of tau may provide "micro-environments" and/or conformations which alter their accessibility and/or reactivity to proteases.