Microtubule-Associated Proteins (MAPs): Life’s Mini Conveyor Belts

As their name suggests, microtubule-associated proteins (MAPs) are a family of proteins that reversibly interact with microtubules, a critical component of the cellular cytoskeleton. Composed of the protein tubulin, microtubules provide structural support, contribute to cell motility and mitosis, and serve as "conveyor belts" that transport vesicles, organelles, and other cellular components throughout the cytoplasm¹. MAPs are a heterogeneous group of proteins that were originally identified in brain extracts based on their ability to co-purify with tubulin². The "classic" and best-characterized MAPs are the structural proteins that promote tubulin polymerization and stabilize microtubules, and are regulated by phosphorylation³. In addition, the term MAP is also often extended to include enzymatically-active proteins such as the microtubule motors kinesin and dynein that move along microtubules by hydrolyzing ATP.

On the basis of their microtubule interaction motif, most structural MAPs belong to either the MAP1A/1B or the MAP2/Tau family, and key players are described below^4,5. MAP1A is predominantly expressed in the dendrites of adult neurons where it is important for dendrite formation, while MAP1B is mainly expressed in the axons of developing neurons and plays a role in axonal outgrowth⁶. Another member of the MAP1A/1B family, MAP1S, is expressed in a variety of neuronal and non-neuronal tissues where it anchors the microtubule-organizing center to the centrosome during mitosis^7,8. MAP2 is preferentially expressed in neurons, is important for crosslinking adjacent microtubules, and shares some functional redundancy with MAP1B⁹. It also interacts with F-actin, in a process thought to promote neurite outgrowth¹⁰. Tau is primarily found in axons and also shares some redundancy with MAP1B4. A hyperphosphorylated form of tau has been implicated in the pathology of Alzheimer disease¹¹. The MAP2/Tau family also includes the neuronal and non-neuronal MAP4, important for cell division¹².