The newly identified augmin complex is required in the assembly of the acentrosomal microtubule assay in higher plants
Zhaosheng Kong^{1, 2, *},Yuh-Ru Julie Lee¹and Bo Liu¹

¹Department of Plant Biology, University of California, Davis, California.
²State Key Laboratory of Plant Genomics, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100101, China.
*E-mail:zskong@im.ac.cn, Tel/ Fax: +86-10-64807397

Abstract
In higher plant cells, microtubules (MTs) are nucleated and organized in a centrosome-independent manner. It is unclear whether augmin-dependent mechanisms underlie spindle MT organization in plant cells as they do in animal cells. When AUGMIN subunit3 (AUG3), which encodes a homolog of animal dimγ-tubulin 3/human augmin-like complex, subunit 3, was disrupted in Arabidopsis thaliana, gametogenesis frequently failed due to defects in cell division. Compared with the controlmicrospores, which formed bipolar spindles at the cell periphery, the mutant cells often formed peripheral half spindlesthat only attached to condensed chromosomes or formed elongated spindles with unfocused interior poles. In addition,defective cells exhibited disorganized phragmoplast MT arrays, which caused aborted cytokinesis. The resulting pollengrains were either shrunken or contained two nuclei in an undivided cytoplasm. AUG3 was localized along MTs in thespindle and phragmoplast, and its signal was pronounced in anaphase spindle poles. An AUG3-green fluorescent proteinfusion exhibited a dynamic distribution pattern, similar to that of the γ-tubulin complex protein.By usingrobustimmunoprecipitation and mass spectrometry, we further identified the entire plant augmin complex that contains eight subunits. Among them, AUG1 to AUG6 share low sequencesimilarity with their animal counterparts, but AUG7 and AUG8 share homology only with proteins of plant origin. We conclude that assembly ofMT arrays during plant mitosis depends on the augmin complex, which includes two plant-specific subunits.