FUNCTION: 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.  SUBCELLULAR LOCATION: Cytoplasm; cytosol. Cell membrane. Mostly found in the axons of neurons, in the cytosol and in association with plasma membrane components. ALTERNATIVE PRODUCTS: 8 named isoforms produced by alternative splicing. Additional isoforms seem to exist. Isoforms differ from each other by the presence or absence of up to 5 of the 15 exons. One of these optional exons contains the additional tau/MAP repeat. TISSUE SPECIFICITY: Expressed in neurons. Isoform PNS-tau is expressed in the peripheral nervous system while the others are expressed in the central nervous system. DEVELOPMENTAL STAGE: Four-repeat (type II) tau is expressed in an adult-specific manner and is not found in fetal brain, whereas three-repeat (type I) tau is found in both adult and fetal brain. DOMAIN: The tau/MAP repeat binds to tubulin. In Alzheimer disease, the neuronal cytoskeleton in the brain is progressively disrupted and replaced by tangles of paired helical filaments  and straight filaments, mainly composed of hyperphosphorylated forms of Microtubule-associated protein Tau. Defects in Microtubule-associated protein Tau are a cause of frontotemporal dementia and parkinsonism linked to chromosome 17, as well as a number of other neurodegenerative diseases.