Secreted tau is cleaved at the C-terminal.
(A) Full-length recombinant human tau protein (rTau4R) was still detectable in the culture medium after being added to control Hela cells for 48 hrs (arrow, M+cells). rTau4R was less degraded when it was added to M without cells (M) (B) Secreted tau is not cleaved at the N-terminal as revealed by the anti-tau antibody Tau12 directed against the 9–18 a.a. A Tau12-positive band was detected in both L and M prepared from Hela cells overexpressing GFP-tau4R corresponding to full-length and cleaved tau (upper and lower arrows). (C) GFP tag inserted at the N-terminal of tau was detected in tau present in both L and M (upper and lower arrows). (D) The microtubule-binding domain of tau was not cleaved in tau secreted by Hela cells as revealed by the K9JA antibody (lower arrow). (E) The band found at 75 kDa in both L and M was not detected by the antibody Tau46 that recognizes the peptidic sequence located between L428 and L441. Only full-length tau present in L was detected with this antibody (arrow). A non-specific band at ∼100 kDa was noted with the antibody Tau46 in M. The pattern of each antibody was analyzed at least in 3 sets of experiments.