A heat-stable protein essential for microtubule assembly, designated tau (τ), has been isolated. Tau is present in association with tubulin purified from porcine brain and is separated from tubulin by ion exchange chromatography on phosphocellulose. In the absence of tau, tubulin exists as a 6S dimer, which cannot assemble into microtubules. Addition of tau restores tubule-forming capacity. Under nonpolymerizing conditions, tau converts 6S dimers to 36S rings, which are intermediates in microtubule formation. Tau appears to activate the 6S tubulin dimer for polymerization. The unique ability of tau to restore normal features of in vitro microtubule assembly suggests it is a major regulator of microtubule formation in cells. Microtubules are a major fibrous system in eukaryotic cells, composed of tubulin, a 6S dimer of α and β tubulin. They are involved in various cellular processes, including cell division, intracellular transport, and maintaining cell shape. Microtubules are stable in some structures like cilia but ephemeral in others. The assembly and disassembly of microtubules are regulated by factors such as GTP, Ca²⁺, and temperature. The discovery of tau provides evidence that a protein other than tubulin is essential for microtubule assembly. Tau is heat-stable, does not bind colchicine, and is removed from tubulin by phosphocellulose chromatography. Tau is essential for the assembly of 6S tubulin into 36S rings and microtubules. It acts on the 6S tubulin dimer, activating it for polymerization. Tau is likely a significant regulatory element for microtubule assembly in vivo. The study also suggests that tau may interact with tubulin and modulate its function, and further research is needed to determine the exact stoichiometry of tau to 6S subunits in microtubules. The role of tau in vivo remains to be fully elucidated, but its ability to induce polymerization in vitro suggests it plays a crucial role in microtubule formation.A heat-stable protein essential for microtubule assembly, designated tau (τ), has been isolated. Tau is present in association with tubulin purified from porcine brain and is separated from tubulin by ion exchange chromatography on phosphocellulose. In the absence of tau, tubulin exists as a 6S dimer, which cannot assemble into microtubules. Addition of tau restores tubule-forming capacity. Under nonpolymerizing conditions, tau converts 6S dimers to 36S rings, which are intermediates in microtubule formation. Tau appears to activate the 6S tubulin dimer for polymerization. The unique ability of tau to restore normal features of in vitro microtubule assembly suggests it is a major regulator of microtubule formation in cells. Microtubules are a major fibrous system in eukaryotic cells, composed of tubulin, a 6S dimer of α and β tubulin. They are involved in various cellular processes, including cell division, intracellular transport, and maintaining cell shape. Microtubules are stable in some structures like cilia but ephemeral in others. The assembly and disassembly of microtubules are regulated by factors such as GTP, Ca²⁺, and temperature. The discovery of tau provides evidence that a protein other than tubulin is essential for microtubule assembly. Tau is heat-stable, does not bind colchicine, and is removed from tubulin by phosphocellulose chromatography. Tau is essential for the assembly of 6S tubulin into 36S rings and microtubules. It acts on the 6S tubulin dimer, activating it for polymerization. Tau is likely a significant regulatory element for microtubule assembly in vivo. The study also suggests that tau may interact with tubulin and modulate its function, and further research is needed to determine the exact stoichiometry of tau to 6S subunits in microtubules. The role of tau in vivo remains to be fully elucidated, but its ability to induce polymerization in vitro suggests it plays a crucial role in microtubule formation.