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MAP2 and tau bind longitudinally along the outer ridges of microtubule protofilaments
Jawdat Al-Bassam1, Rachel S Ozer, Daniel Safer
1Department of Cell Biology, Scripps Research Institute, La Jolla, CA 92037, USA.
Abstract:
MAP2 and tau exhibit microtubule-stabilizing activities that are implicated in the development and maintenance of neuronal axons and dendrites. The proteins share a homologous COOH-terminal domain, composed of three or four microtubule binding repeats separated by inter-repeats (IRs). To investigate how MAP2 and tau stabilize microtubules, we calculated 3D maps of microtubules fully decorated with MAP2c or tau using cryo-EM and helical image analysis. Comparing these maps with an undecorated microtubule map revealed additional densities along protofilament ridges on the microtubule exterior, indicating that MAP2c and tau form an ordered structure when they bind microtubules. Localization of undecagold attached to the second IR of MAP2c showed that IRs also lie along the ridges, not between protofilaments. The densities attributable to the microtubule-associated proteins lie in close proximity to helices 11 and 12 and the COOH terminus of tubulin. Our data further suggest that the evolutionarily maintained differences observed in the repeat domain may be important for the specific targeting of different repeats to either alpha or beta tubulin. These results provide strong evidence suggesting that MAP2c and tau stabilize microtubules by binding along individual protofilaments, possibly by bridging the tubulin interfaces.
Insights
Microtubule-associated proteins MAP2 and tau stabilize neuronal microtubules by binding along protofilaments. Their homologous COOH-terminal domains form ordered structures, potentially bridging tubulin interfaces for stability.
Area of Science:
- Neuroscience
- Cell Biology
- Structural Biology
Background:
- Microtubule-associated protein 2 (MAP2) and tau are crucial for neuronal development and maintenance.
- Both proteins share a homologous COOH-terminal domain with microtubule binding repeats and inter-repeats (IRs).
- Understanding their stabilization mechanisms is key to neuronal function.
Purpose of the Study:
- To elucidate the structural basis of microtubule stabilization by MAP2c and tau.
- To investigate the binding sites and interactions of MAP2c and tau on microtubules.
- To explore the role of homologous domains in microtubule association.
Main Methods:
- Cryo-electron microscopy (cryo-EM) and helical image analysis were used to generate 3D maps.
- Microtubules fully decorated with MAP2c or tau were compared to undecorated microtubules.
- Undecagold localization was employed to pinpoint the position of MAP2c inter-repeats.
Main Results:
- MAP2c and tau form ordered structures on the microtubule exterior along protofilament ridges.
- Inter-repeats (IRs) of MAP2c were localized along these ridges, not between protofilaments.
- Bound proteins are in proximity to tubulin helices 11 and 12 and the COOH terminus, suggesting protofilament binding and potential bridging of tubulin interfaces.
Conclusions:
- MAP2c and tau stabilize microtubules by binding along individual protofilaments.
- The homologous repeat domains are critical for this ordered interaction.
- Differences in repeat domains may mediate specific targeting to alpha or beta tubulin.