Researchers in the labs of Dr. ZHU Xueliang, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, and Dr. ZHENG Yixian, Carnegie Institution for Science, USA, jointly found that BuGZ, a mitotic regulator containing low complexity domains, undergoes phase transition or coacervation to promote assembly of both spindle and spindle matrix through hydrophobic intermolecular interactions.  

Purified BuGZ forms liquid droplets of varying size in physiological buffers. This process is reversible and is protein concentration- and temperature-dependent. The droplets can enrich free tubulin and stimulate microtubule (MT) polymerization and bundling. The coacervation can also occur on MTs, the presence of which markedly reduces the critical concentration of BuGZ phase transition.  

More importantly, BuGZ coacervation and its binding to MTs or tubulin are critical for assembly of spindle and spindle matrix, as revealed in Xenopus egg extract and mammalian cells.  

Earlier collaborative studies by the two groups have identified BuGZ as a spindle matrix protein important for efficient and accurate mitosis through the spindle checkpoint protein Bub3 (Jiang et al., Dev Cell, 2014). Following this previous study, Dr. JIANG Hao, who was then a graduate student and is now a joint postdoctoral fellow, accomplishes this work together with his colleagues.  

This study, published online by Cell on September 17^th, 2015, was supported by research funds from the National Natural Science Foundation of China,  the Ministry of Science and Technology of China, the Chinese Academy of Sciences, and the NIH.   

Figure. Models for BuGZ phase transition in vitro or in vivo. The greyscale micrograph shows droplets formed by BuGZ in vitro. The color one shows BuGZ (red), MTs (green), and chromosome (blue) in a mitotic HeLa cell. (Image by Prof. ZHU Xueliang`s group) 

CONTACT:
ZHU Xueliang, Principal Investigator
Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences,
Shanghai 200031, China
Phone: 86-21-54921406
E-mail: xlzhu@sibcb.ac.cn