Inhibitor‐3 ensures bipolar mitotic spindle attachment by limiting association of SDS 22 with kinetochore‐bound protein phosphatase‐1

• Published in: The EMBO journal 2014-11, Vol.33 (22), p.2704-2720
• Main Authors: Eiteneuer, Annika, Seiler, Jonas, Weith, Matthias, Beullens, Monique, Lesage, Bart, Krenn, Veronica, Musacchio, Andrea, Bollen, Mathieu, Meyer, Hemmo
• Format: Article
• Language: English
• ISSN: 0261-4189; 1460-2075
• DOI: 10.15252/embj.201489054

Faithful chromosome segregation during mitosis is tightly regulated by opposing activities of Aurora B kinase and protein phosphatase‐1 ( PP 1). PP 1 function at kinetochores has been linked to SDS 22, but the exact localization of SDS 22 and how it affects PP 1 are controversial. Here, we confirm that SDS 22 is required for PP 1 activity, but show that SDS 22 does not normally localize to kinetochores. Instead, SDS 22 is kept in solution by formation of a ternary complex with PP 1 and inhibitor‐3 (I3). Depletion of I3 does not affect the amount of PP 1 at kinetochores but causes quantitative association of SDS 22 with PP 1 on KNL 1 at the kinetochore. Such accumulation of SDS 22 at kinetochores interferes with PP 1 activity and inhibits Aurora B threonine‐232 dephosphorylation, which leads to increased Aurora B activity in metaphase and persistence in anaphase accompanied with segregation defects. We propose a model in which I3 regulates an SDS 22‐mediated PP 1 activation step in solution that precedes SDS 22 dissociation and transfer of PP 1 to kinetochores, and which is required for PP 1 to efficiently antagonize Aurora B. image Reversible protein phosphorylation at kinetochores is essential for ensuring correct spindle attachment and chromosome segregation. Antagonism of Aurora B kinase action by protein phosphatase‐1 is controlled by intricate interplay of two PP 1‐interacting regulators. SDS 22 is essential to activate KNL 1‐bound PP 1 at the kinetochore so that it can antagonize Aurora B during mitotic chromosome biorientation. SDS 22 itself does not normally localize to kinetochores and increased SDS 22 binding to PP 1 at the kinetochore in fact inhibits PP 1 activity. Inhibitor‐3 (I3) forms a complex with SDS 22‐ PP 1 in solution and prevents association of SDS 22 to KNL 1‐bound PP 1 to ensure PP 1 activity at the kinetochore. SDS 22 in cooperation with I3 may act as a chaperone that activates PP 1 in solution prior to recruitment to the kinetochore.