Augmin’s Role in Microtubule Generation in Plants

During mitosis and meiosis, microtubules (MTs) form bipolar spindles that facilitate chromosome segregation. In animals, spindle MTs have historically been thought of as originating from the centrosomes, but MTs also nucleate from other MTs and near chromatin (Goshima and Kimura, 2010). Flowering plants do not contain centrosomes, but it is becoming clear that cell division in plants and animals is more similar than previously thought (Lloyd and Chan, 2006).

The augmin complex binds spindle MTs as well as the γ-tubulin ring complex (γ-TuRC) and is important for spindle MT generation without centrosome involvement. Augmin was discovered in Drosophila melanogaster, but a homolog of augmin subunit3 (AUG3) was recently shown to be needed for Arabidopsis thaliana spindle and phragmoplast formation (Ho et al., 2011). Thus, augmin’s role in spindle formation may be conserved, consistent with the generic similarities between plant and animal cell division.

Two new articles provide a more comprehensive view of plant augmins. Nakaoka et al. (pages 1478–1493) elegantly adapted an inducible RNA interference system for use in the moss Physcomitrella patens and examined the roles of essential proteins using three-dimensional time-lapse confocal microscopy of live cells. Among the proteins they knocked down was Aug3. The Aug3 knockdown cells exhibited prolonged mitosis and improper spindle and phragmoplast formation. In addition, MT signal intensity was reduced (see figure), suggesting that loss of Aug3 might result in fewer sites for MT nucleation. Fluorescence recovery after photobleaching and analysis of Aug3, as well as of the γ-TuRC, supported a model in which augmin activates the γ-TuRC to generate spindle and phragmoplast MTs.

Physcomitrella caulonemal cells expressing green fluorescent protein–tubulin (green) and histone–red fluorescent protein (red). Merged images, below. Time from nuclear envelope breakdown is indicated. Arrowhead indicates misaligned chromosomes in the Aug3-depleted line. Bar = 10 μm. (Image from Figure 4 of Nakaoka et al. [2012].)

Hotta et al. (pages 1494–1509) used a biochemical approach to identify augmin subunits from Arabidopsis plants expressing a functional, myc-tagged version of AUG3. In addition to the previously identified AUG1 and AUG3, they isolated four new Arabidopsis homologs of animal augmins: AUG2, 4, 5, and 6, all of which are also found in Physcomitrella. Independent T-DNA insertions in AUG1, 2, 4, and 5 were lethal, with microgametophytes for each line displaying disrupted spindle and phragmoplast MT organization. Localization data were consistent with the formation of an augmin complex that associates with MTs and γ-tubulin in spindles and phragmoplasts. Hotta et al. also discovered two plant-specific augmin subunits, AUG7 and AUG8. AUG7, for which there is a moss homolog, is likely a bona fide member of the Arabidopsis augmin complex. The aug7 T-DNA insertion mutant displayed elongated spindles, abnormal phragmoplast MT arrays, and reduced γ-tubulin localization and MT signal intensities in both structures, strikingly similar phenotypes to those Nakaoka et al. observed in their Aug3-depleted moss cells.

Collectively, these articles support a role for augmin in plant acentrosomal MT generation during mitosis and elucidate the makeup of the augmin complex in plants.