SPINDOC functions as a mitotic molecular counter to coordinate spatiotemporal CENP-A assembly

Cell cycle-dependent maintenance of centromere protein A (CENP-A) levels and its spatiotemporal assembly are essential for centromere propagation. CENP-A synthesis peaks in late G2, while its assembly occurs during late telophase/early G1. We have previously shown that phosphorylation of CENP-A at Ser68 by CDK1-cyclin B during mitosis impairs its binding to holiday junction recognition protein (HJURP) and facilitates DCAF11-dependent polyubiquitination and degradation. However, the mechanisms governing CENP-A^pS68 stability remain elusive. Here, we demonstrate that spindlin interactor and repressor of chromatin binding (SPINDOC), as an M-phase-specific maintenance factor for CENP-A assembly, binds and stabilizes CENP-A^pS68 by antagonizing DCAF11-dependent polyubiquitination. In addition, SPINDOC bridges CENP-A^pS68 to HJURP, ensuring that CENP-A^pS68 is poised for subsequent deposition at centromeres. Interestingly, SPINDOC promotes liver cancer development in vitro and in vivo, and alterations in its levels disrupt CENP-A^pS68 homeostasis, resulting in chromosomal instability. Together, this study identifies SPINDOC as a mitotic molecular counter of newly synthesized CENP-A, coordinating its spatiotemporal assembly by presenting it to HJURP.