Stabilizing the microtubule scaffold

Combining stability with movement is a perpetual challenge for engineers. Linda Sandblad, Damian Brunner, Andreas Hoenger (EMBL, Heidelberg, Germany), and colleagues now report just such an engineering feat for a microtubule binding protein. Mal3p, they show, binds in such a way as to both stabilize microtubules and allow free-flowing transportation along their length.

Microtubules are not just the cell's scaffolding, they are also the highways for transporting factors and vesicles. To perform their various functions, microtubules interact with a number of motor proteins and other microtubule-associated proteins (MAPs), the most conserved of which are the end binding proteins, such as EB1.

Despite its name, the precise mode in which EB1 binds to microtubules was unknown. Light microscopy had revealed an accumulation of the yeast EB1 homologue, Mal3p, at the microtubule plus end, but also a faint signal along the microtubule length. Sandblad et al. used metal shadowing electron microscopy, in which a fine layer of metal is sprayed onto the sample, to look more closely at Mal3p binding. Like snow blown onto a tree, the metal builds up and brings the topography of the sample into sharp relief.

The team observed that Mal3p molecules aligned along the length of the microtubule but typically all in a single one of the many surface grooves. This, they showed, was the microtubule's seam.

The seam is formed by the closure of tubulin lattice sheets—like the edges of a sheet of paper rolled into the shape of a tube. The unique presence of Mal3p at these seams suggests Mal3p acts like sticking tape to hold the two edges of the sheet together. The binding of Mal3p thus provides stability and yet leaves the rest of the microtubule surface free for proteins to motor along to their cellular destinations.

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