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Vol. 10, Issue 3, 727-740, March 1999
Max-Planck-Unit for Structural Molecular Biology, D-22603 Hamburg,
Germany
The differentiation of neurons and the outgrowth of neurites
depends on microtubule-associated proteins such as tau protein. To
study this process, we have used the model of Sf9 cells, which allows
efficient transfection with microtubule-associated proteins (via
baculovirus vectors) and observation of the resulting neurite-like extensions. We compared the phosphorylation of tau23 (the
embryonic form of human tau) with mutants in which critical
phosphorylation sites were deleted by mutating Ser or Thr residues into
Ala. One can broadly distinguish two types of sites, the KXGS motifs in the repeats (which regulate the affinity of tau to microtubules) and
the SP or TP motifs in the domains flanking the repeats (which contain
epitopes for antibodies diagnostic of Alzheimer's disease). Here we
report that both types of sites can be phosphorylated by endogenous
kinases of Sf9 cells, and that the phosphorylation pattern of the
transfected tau is very similar to that of neurons, showing that Sf9
cells can be regarded as an approximate model for the neuronal balance
between kinases and phosphatases. We show that mutations in the repeat
domain and in the flanking domains have opposite effects. Mutations of
KXGS motifs in the repeats (Ser262, 324, and 356) strongly inhibit the
outgrowth of cell extensions induced by tau, even though this type of
phosphorylation accounts for only a minor fraction of the total
phosphate. This argues that the temporary detachment of tau from
microtubules (by phosphorylation at KXGS motifs) is a necessary
condition for establishing cell polarity at a critical point in space
or time. Conversely, the phosphorylation at SP or TP motifs represents the majority of phosphate (>80%); mutations in these motifs cause an
increase in cell extensions, indicating that this type of
phosphorylation retards the differentiation of the cells.
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