PP2A-B56 isoform specificity at the centromere and kinetochore
At least three major mitotic processes are regulated by the PP2A-B56 phosphatase complex: the Spindle Assembly Checkpoint (SAC), kinetochore-microtubule attachments and sister chromatid cohesion. We show here that these key functions of PP2A-B56, which require its localization to either the kinetochore or centromere, are split between distinct subsets of B56 isoforms. PP2A-B56γ and PP2A-B56δ localize to the outer kinetochore (via BUBR1), whereas PP2A-B56α and PP2A-B56ε localize to the centromere (via Sgo2). The differential localization observed is due to a difference in affinity for the receptors: PP2A-B56γ has a reduced affinity for Sgo2 compared to PP2A-B56α and, vice versa, PP2A-B56α has a reduced affinity for BUBR1 compared to PP2A-B56γ. Given that the known binding interfaces for both BUBR1 and Sgo2 are highly conserved in all B56 isoform, we generated a series of chimeras between B56α and B56γ to uncover isoform specific interactions. This led to the identifications of two distinct regions within B56α and B56γ that regulate the binding to Sgo2 and BUBR1. Furthermore, site directed mutagenesis has revealed that proper holoenzyme assembly has a role in regulating the localization of B56: it is needed for centromeric accumulation and it interferes with kinetochore accumulation of B56α. We will present a model to explain how this differential localization could be linked to post-translational modifications of PP2AC. Together, these results help to clarify how individual PP2A-B56 isoforms achieve subcellular specificity during mitosis.
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