RESEARCH ARTICLE CM1-driven assembly and activation of yeast g-tubulin small complex underlies microtubule nucleation Axel F Brilot1, Andrew S Lyon1†‡, Alex Zelter2, Shruthi Viswanath3§, Alison Maxwell1, Michael J MacCoss4, Eric G Muller2, Andrej Sali3, Trisha N Davis2, David A Agard1* 1Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, United States; 2Department of Biochemistry, University of Washington, Seattle, United States; 3Department of Bioengineering and Therapeutic Sciences, University of California at San Francisco, San Francisco, United States; 4Department of Genome Sciences, University of Washington, Seattle, United States *For correspondence:
[email protected] Abstract Microtubule (MT) nucleation is regulated by the g-tubulin ring complex (gTuRC), Present address: †Department conserved from yeast to humans. In Saccharomyces cerevisiae, gTuRC is composed of seven of Biophysics and Howard identical g-tubulin small complex (gTuSC) sub-assemblies, which associate helically to template MT Hughes Medical Institute, growth. gTuRC assembly provides a key point of regulation for the MT cytoskeleton. Here, we University of Texas Southwestern combine crosslinking mass spectrometry, X-ray crystallography, and cryo-EM structures of both Medical Center, Dallas, United monomeric and dimeric gTuSCs, and open and closed helical gTuRC assemblies in complex with ‡ States; Howard Hughes Medical Spc110p to elucidate the mechanisms of gTuRC assembly. gTuRC assembly is substantially aided by Institute, University of Texas the evolutionarily conserved CM1 motif in Spc110p spanning a pair of adjacent gTuSCs. By Southwestern Medical Center, providing the highest resolution and most complete views of any gTuSC assembly, our structures Dallas, United States; § National allow phosphorylation sites to be mapped, surprisingly suggesting that they are mostly inhibitory.