A new MT is nucleated (white arrowhead) by a γ-TuC (green) that sits on an existing MT bundle.

Microtubule (MT) arrays form, say Janson et al. (page 297), when MT nucleators stick to existing MTs. This creates new nucleation sites for more MTs, thanks to the activities of γ-tubulin and a MT-associated protein called mto2p.Fission yeast have a simple, consistent interphase MT arrangement composed of four bundles per cell. The two antiparallel MTs that make up each bundle overlap at their minus ends at the interphase microtubule organizing center (iMTOC), which attaches to the nuclear envelope.

The nuclear envelope is also where mto2p has been found. The new results suggest that mto2p organizes interphase MTs by recruiting a complex of γ-tubulin and its associated proteins (γ-TuC) to existing MTs. mto2p colocalized with γ-TuCs on MTs, particularly at iMTOCs. It was at these colocalization sites where most new MTs were born, suggesting that mto2p may activate the γ-TuC once it binds to a MT.

The emergence of a new MT from an old one helps to form the antiparallel arrangement of the bundles. The minus ends of new MTs consistently pointed away from the existing iMTOC. The basis for the preferential growth direction is not clear, but may be due to bundling and stabilization by Ase1p, which may prefer to bind to antiparallel MTs.

Mutant cells lacking mto2p had only one bundle instead of the usual four. In these cells, γ-TuCs were not found on MTs. The mutants were thus unable to nucleate new MTs after their arrays were depolymerized.

The more complex linear interphase arrays common in epithelial cells, neurons, and plant cells may also be formed by MT-bound γ-TuCs. The spindle midzone is another site of antiparallel MT overlap that harbors γ-TuCs. Centrosome-independent spindle formation may thus also rely on MT-bound γ-TuCs.