A ring for all: γ-tubulin-containing nucleation complexes in acentrosomal plant microtubule arrays

The construction of dynamic polar microtubules from 13 protofilaments consisting of alpha-tubulin and beta-tubulin heterodimers requires a preformed nucleation seed that specifies subcellular localization and timing of microtubule polymerization in vivo. An evolutionarily conserved gamma-tubulin-containing ring complex is recruited to the lateral wall of preexisting microtubules or outer nuclear membranes in plant cells, and is then activated as a template for new microtubules of defined geometry. Specific regulators are thought to target/activate the ring complex to nucleate nascent microtubules in distinct polymerization patterns, as seen in interphase and mitotic arrays. The augmin complex, which was initially identified in metazoan cells, recruits the ring complex to plant mitotic microtubules, where new polymers are abundantly generated at shallow angles.