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. Interacts with MARK1, MARK2, MARK3 AND MARK4 (By similarity). Interacts with SQSTM1 when polyubiquitinated (PubMed:15953362). Interacts with PSMC2 through SQSTM1 (PubMed:15953362). Interacts with FKBP4 (By similarity). Binds to CSNK1D (By similarity). Interacts with SGK1 (By similarity). Interacts with EPM2A; the interaction dephosphorylates MAPT at Ser-369 (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. Can be secreted; the secretion is dependent on protein unfolding and facilitated by the cargo receptor TMED10; it results in protein translocation from the cytoplasm into the ERGIC (endoplasmic reticulum-Golgi intermediate compartment) followed by vesicle entry and secretion. Additional isoforms seem to exist. Isoforms differ from each other by the presence or absence of up to 5 of the 14 exons. One of these optional exons contains the additional tau/MAP repeat. Two different C-termini are obtained either by the retention or the splicing of intron 13/14. Expressed in neurons and at a lower level in the liver and kidney. Isoform PNS-tau is expressed in the peripheral nervous system while the others are expressed in the central nervous system. Shorter forms or low molecular weight tau (lMW-tau) are generally expressed at early development stages and longer forms or high molecular weight tau (hMW-tau) in the adult brain. The tau/MAP repeat binds to tubulin. Type I isoforms contain 3 repeats while type II isoforms contain 4 repeats. 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, MARK4), causing detachment from microtubules, and their disassembly (By similarity). Phosphorylated by PHK. Dephosphorylation at several serine and threonine residues by the serine/threonine phosphatase PPP5C. Phosphorylation at Ser-554 by BRSK1 and BRSK2 in neurons affects ability to bind microtubules and plays a role in neuron polarization. Phosphorylation at Ser-188 by SGK1 mediates microtubule depolymerization and neurite formation in hippocampal neurons (By similarity). May be involved in the pathogenesis of cytoplasmic inclusions (as Mallory bodies) in livers of mice chronically intoxicated with Griseofulvin or DDC (3,5-diethoxycarbonyl-2,4-dihydrocollidine), a model for human alcoholic hepatitis. Alteration of Tau (abnormal phosphorylation and cross-linking) could contribute to Mallory bodies formation and disturbance of microtubule function in alcoholic liver disease.