Microtubule-associated proteins (MAPs) reinforce microtubules (MTs). Fourniol et al. provide the first close-up of a MT-bound MAP, clarifying how it adds stability.
The building blocks of MTs are dimers of one α- and one β-tubulin subunit. These dimers line up end to end to build protofilaments, which in turn align side by side in a ring shape to form a hollow MT. MAPs help fasten the protofilaments together. One type of MAP, known as doublecortin (DCX), fortifies MTs that contain 13 protofilaments, the arrangement in almost all cells. However, researchers haven't been able to get a close enough look at MT-bound MAPs to determine how they strengthen the fibers.
Fourniol et al. used cryo-electron microscopy to create the highest-resolution reconstruction of a MT-bound MAP yet. Using kinesin-1 motors linked to the MTs as landmarks, the researchers nailed down where DCX hooked on. The protein targeted the MT surface at intersections where four tubulin dimers join. In this position, DCX stabilizes the structure laterally and lengthwise.
Thirteen-protofilament MTs contain a seam that forms where α- and β-tubulin subunits fall out of register. The seam runs the length of the MT fiber and is a potential weak spot—some studies suggest that MTs can split open along this fault line. However, the researchers found that DCX avoids the seam, suggesting that it might not require extra stabilization after all. Instead, DCX might act as an on/off switch—its binding to MTs ensuring they grow where the cell needs them. Similar studies should reveal how other MAPs show their support for MTs.