Fig. 1.

Schematic and microtubule binding of tau constructs used in this study. A: The longest isoform of tau in the human central nervous system is referred to as hTau40. Boxes represent alternatively spliced exons (E2 and E3), the proline-rich region, and the microtubule-binding repeats (R1–R4). Δ2−18 and 1−421 are constructs containing N-terminal and C-terminal deletions, respectively, and K23 is a construct lacking N-terminal exons and microtubule-binding repeats. Tau6P and Tau6D are isoforms that are identical to canonical tau from a.a. 1−144, at which point alternative splicing introduces 11 unique a.a. (represented by dark boxes) followed by a stop codon. The specific amino acids differ between Tau6P and Tau6D. B: Tau constructs (5 μM) were incubated with microtubule-containing squid optic lobe cytosol as described in Materials and Methods. Microtubule pellets (P) and supernatant (S) fractions were prepared by centrifugation and analyzed by immunoblot using antibodies against tau (Tau5) and tubulin. Note that the K23 tau construct lacking MTBRs is mostly recovered in supernatant fractions, consistent with its weak binding to microtubules. In contrast, hTau40 is recovered in association with microtubules. Neither construct was recovered in pellet fractions when centrifuged in the absence of squid microtubules (data not shown).