Microtubules follow septins’ guidelines

Microtubules follow septins' guidelines T he microtubules inside an epithelial cell undergo dramatic rearrangements when the cell becomes part of a polarized epithelial sheet. On its own, an epithelial cell is fl at, and its microtubules radiate out from the centrosome. But, after forming contacts with neighboring cells, an epithelial cell rises up into a columnar shape, and its microtubules form a more complex network: microtubule bundles run from the top to the bottom of the cell, while mesh-works of shorter tubulin fi laments underlie both the apical and basal membranes. Bowen et al. reveal that this reorganization is guided by another cytoskeleton— the network of fi laments formed by the septin family of GTPases (1). Septins form hetero-oligomers with each other that can assemble into fi laments and other higher-order structures (2), and they have previously been shown to associate with subsets of microtubules (3, 4). " But people haven't paid much attention to the septin–microtubule connection, " says Elias Spiliotis from Drexel University in Phila-delphia, PA. " We know they're associated, but we don't know what this means for microtubule organization and dynamics. " Spiliotis and colleagues decided to investigate how septins affect microtubule re-modeling during the polarization of MDCK epithelial cells (1). When MDCK cells fi rst contact each other, their microtu-bules convert from a radial array into a diverse network that includes long microtu-bule bundles near the nucleus , which Bowen et al. found were associated with septin fi laments. Knocking down septin 2 reduced the appearance of both the septin fi laments and the perinuclear microtubule bundles. In addition, shorter septin fi laments were associated with the ends of peripheral microtubules, which failed to properly target to the cell cortex in the absence of septin 2. Septins might organize perinuclear and peripheral microtubules by regulating their dynamics. Indeed, knocking down septin 2 increased the frequency and duration of microtubule " catastrophes " —when micro-tubules shrink—suggesting that septin fi laments promote the growth of microtubules by inhibiting their depolymerization. Septins also control the direction of microtubule growth. By simultaneously imaging septin fi laments and microtubule plus ends, Bowen et al. found that micro-tubules grew along septin fi laments in both the perinuclear and peripheral regions of the cell. Microtubule ends often collided with septin-decorated microtubules and redirected their growth in line with these fi laments. In the absence of septin 2, microtubules failed to align and …