26.4: Microtubule Associated Proteins (MAPs)
Microtubule function and architecture are regulated by an array of specialized proteins called microtubule-associated proteins or MAPs. These proteins are widespread across different organisms and have conserved protein motifs, like the multi-TOG domain for tubulin binding found in the CLASP family of MAPs. Some MAPs are lineage-specific based on their conserved domains. Their functions depend upon the cytoskeletal architecture and cell type they are located within. In-plant cells, a specific microtubule-associated protein‒ tortifolia, binds with cortical microtubules to regulate organ the orientation and direction of organ growth. On the other hand, tau proteins are specifically associated with microtubules in neurons in animal cells. MAPs were first identified within neurons and were named “classical MAPs''.
Depending on how MAPs regulate microtubules, they are broadly classified as stabilizers, destabilizers, capping proteins, crosslinkers, and cytoskeleton integrator proteins. MAPs are further classified based on where they localize on the microtubules. They are broadly divided into three groups: Lattice-binding proteins, microtubule plus-end trafficking proteins, and minus-end targeting proteins. Lattice binding proteins bind along the filament length instead of the microtubule plus or minus end. Tau and MAP2 found in neurons' axonal and dendritic microtubules belong to the lattice-binding MAPs. Microtubule plus-end trafficking proteins include proteins that target the growing end of the microtubules. Examples include EB1, XMAP-215, and kinesin-13. EB1 and XMAP-215 are microtubule-stabilizing and growth-promoting proteins, while kinesin-13 is a microtubule destabilizer. Minus-end targeting proteins include microtubule formation initiator proteins like the γ-tubulin ring complex (γ-TRC) and capping proteins like the calmodulin regulated spectrin-associated protein family (CAMSAPs) members. CAMSAPs bind to the minus-end of microtubules to stabilize them and prevent the dissociation of the tubulin subunits.
The dynamic structure of microtubules varies throughout the cell cycle. During interphase, the microtubule network transports organelles and vesicles and helps organize the cytoskeleton within the cell. As the cell enters into a dividing phase, the previous microtubule mesh disassembles and reorganizes into mitotic spindles that aid in separating chromosomes and cytokinesis. These functions and variability of microtubules are possible due to the various MAPs present within the cell.
