1. Department of Systems Biology, Harvard Medical School, Boston, Massachusetts 02115, USA
2. Department of Pharmacology and Toxicology, College of Pharmacy, and Arizona Cancer Center, University of Arizona, Tucson, Arizona 85721, USA

Correspondence to: Paul Chang¹ Email: paul_chang2@hms.harvard.edu

The mitotic spindle is typically thought of as an array of microtubules, microtubule-associated proteins and motors that self-organizes to align and segregate chromosomes¹. The major spindle components consist of proteins and DNA, the primary structural elements of the spindle¹. Other macromolecules including RNA and lipids also associate with spindles, but their spindle function, if any, is unknown. Poly(ADP-ribose) (PAR) is a large, branched, negatively charged polymeric macromolecule whose polymerization onto acceptor proteins is catalysed by a family of poly(ADP-ribose) polymerases (PARPs)². Several PARPs localize to the spindle in vertebrate cells, suggesting that PARPs and/or PAR have a role in spindle function². Here we show that PAR is enriched in the spindle and is required for spindle function—PAR hydrolysis or perturbation leads to rapid disruption of spindle structure, and hydrolysis during spindle assembly blocks the formation of bipolar spindles. PAR exhibits localization dynamics that differ from known spindle proteins and are consistent with a low rate of turnover in the spindle. Thus, PAR is a non-proteinaceous, non-chromosomal component of the spindle required for bipolar spindle assembly and function.

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