Spindle assembly requires a large complex that includes essential RNAs, say Michael Blower, Maxence Nachury, Rebecca Heald, and Karsten Weis (University of California, Berkeley, CA).
The team was searching in frog egg extracts for spindle assembly factors—specifically those downstream of the Ran GTPase. Ran activated by a chromatin-bound GTP exchange factor displaces importin β, an inhibitor of spindle assembly. Known factors downstream of Ran all bind importin β indirectly, via its partner importin α.
Depleting extracts using a mutant importin β gave a nonfunctional extract even though proteins that bind importin α remained. What was missing was Rae1. This importin β-binding protein was previously associated with mRNA export in yeast, and full spindle-promoting activity of Rae1 required a complex of 10 or more proteins and a host of RNAs. Without the RNA, the complex lost several of its proteins and some of its activity. Clusters of RNA were visible in the spindle, and RNase treatment of frog egg extracts inhibited spindle assembly.
“Everyone believed that the spindle was a purely protein-based machine,” says Weis. Identifying the new RNAs will help determine whether they simply hold together a complex or are more active, as in ribosomes and spliceosomes.