Promotes microtubule assembly and stability, and might be involved in the establishment and maintenance of neuronal polarity. The C-terminus binds axonal microtubules while the N-terminus binds neural plasma membrane components, suggesting that tau functions as a linker protein between both. Axonal polarity is predetermined by tau localization (in the neuronal cell) in the domain of the cell body defined by the centrosome. The short isoforms allow plasticity of the cytoskeleton whereas the longer isoforms may preferentially play a role in its stabilization (By similarity). Interacts with MARK1, MARK2, MARK3 AND MARK4 (By similarity). Interacts with SQSTM1 when polyubiquitinated (By similarity). Interacts with PSMC2 through SQSTM1 (By similarity). Interacts with FKBP4 (By similarity). Binds to CSNK1D (By similarity). Interacts with SGK1 (By similarity). Interacts with EPM2A; the interaction dephosphorylates MAPT at Ser-396 (By similarity). Interacts with PIN1 (By similarity). Interacts with LRRK2 (By similarity). Interacts with LRP1, leading to endocytosis; this interaction is reduced in the presence of LRPAP1/RAP (By similarity). Mostly found in the axons of neurons, in the cytosol and in association with plasma membrane components. The tau/MAP repeat binds to tubulin. Polyubiquitinated. Requires functional TRAF6 and may provoke SQSTM1-dependent degradation by the proteasome (By similarity). Phosphorylation at various serine and threonine residues in S-P or T-P motifs by proline-directed protein kinases (PDPK1, CDK1, CDK5, GSK3, MAPK) (a few sites per protein in interphase, more in mitosis), and at serine residues in K-X-G-S motifs by MAP/microtubule affinity-regulating kinase (MARK1, MARK2, MARK3 or MARK4), causing detachment from microtubules, and their disassembly (By similarity). Phosphorylation at Ser-579 by BRSK1 and BRSK2 in neurons affects ability to bind microtubules and plays a role in neuron polarization. Phosphorylated by PHK. Dephosphorylation at several serine and threonine residues by the serine/threonine phosphatase PPP5C (By similarity). Phosphorylation at Ser-214 by SGK1 mediates microtubule depolymerization and neurite formation in hippocampal neurons (By similarity).