Phosphorylation affects the ability of tau protein to promote microtubule assembly. Tau is a family of proteins that can copolymerize with tubulin to form microtubules. When tau is dephosphorylated, it undergoes a conformational change and promotes more rapid and extensive microtubule polymerization. A kinase in partially purified microtubule protein can rephosphorylate tau. Tau was discovered as a non-tubulin component of microtubule protein. It is an important component of microtubules in vivo and plays a role in regulating microtubule assembly and disassembly. Phosphorylation of tau may control its activity, as it has been shown to affect microtubule polymerization. The phosphorylation state of tau can be reversed, and tau can be dephosphorylated and rephosphorylated. Radiolabeling studies showed that phosphorylation of tau leads to a change in electrophoretic mobility. Dephosphorylated tau promotes more rapid and extensive microtubule polymerization than untreated tau. Electron microscopy confirmed that both tau preparations induce microtubule formation. The two phosphorylation states of tau have different electrophoretic mobilities. The shift in mobility is likely due to the incorporation of a single phosphate. The second mode of phosphorylation does not cause a significant mobility shift. The kinase responsible for the transformation from state I to state II is distinct from the kinase responsible for additional phosphorylation. Phosphorylation of tau may affect its ability to stabilize microtubules. The role of tau in the cytoskeleton is complex, and its phosphorylation may be involved in regulating microtubule assembly and disassembly. The study shows that tau can exist in two states, and its phosphorylation state affects its ability to promote microtubule assembly. The exact kinases involved in tau phosphorylation are not yet known. The study highlights the importance of phosphorylation in regulating tau function and microtubule assembly.Phosphorylation affects the ability of tau protein to promote microtubule assembly. Tau is a family of proteins that can copolymerize with tubulin to form microtubules. When tau is dephosphorylated, it undergoes a conformational change and promotes more rapid and extensive microtubule polymerization. A kinase in partially purified microtubule protein can rephosphorylate tau. Tau was discovered as a non-tubulin component of microtubule protein. It is an important component of microtubules in vivo and plays a role in regulating microtubule assembly and disassembly. Phosphorylation of tau may control its activity, as it has been shown to affect microtubule polymerization. The phosphorylation state of tau can be reversed, and tau can be dephosphorylated and rephosphorylated. Radiolabeling studies showed that phosphorylation of tau leads to a change in electrophoretic mobility. Dephosphorylated tau promotes more rapid and extensive microtubule polymerization than untreated tau. Electron microscopy confirmed that both tau preparations induce microtubule formation. The two phosphorylation states of tau have different electrophoretic mobilities. The shift in mobility is likely due to the incorporation of a single phosphate. The second mode of phosphorylation does not cause a significant mobility shift. The kinase responsible for the transformation from state I to state II is distinct from the kinase responsible for additional phosphorylation. Phosphorylation of tau may affect its ability to stabilize microtubules. The role of tau in the cytoskeleton is complex, and its phosphorylation may be involved in regulating microtubule assembly and disassembly. The study shows that tau can exist in two states, and its phosphorylation state affects its ability to promote microtubule assembly. The exact kinases involved in tau phosphorylation are not yet known. The study highlights the importance of phosphorylation in regulating tau function and microtubule assembly.