Summary Haspin phosphorylates histone H3 at Thr-3 (H3T3ph) during mitosis [1, 2], providing a chromatin binding site for the chromosomal passenger complex (CPC) at centromeres to regulate chromosome segregation [3–5]. H3T3ph becomes increasingly focused at inner centromeres during prometaphase [1, 2], but little is known about how its level or location and the consequent chromosomal localization of the CPC are regulated. In addition, CPC binding to Shugoshin proteins contributes to centromeric Aurora B localization [5, 6]. Recruitment of the Shugoshins to centromeres requires the phosphorylation of Histone H2A at T120 (H2AT120ph) by the kinetochore kinase Bub1 [7], but the molecular basis for the collaboration of this pathway with H3T3ph has been unclear. Here, we show that Aurora B phosphorylates Haspin to promote generation of H3T3ph, and that Aurora B kinase activity is required for normal chromosomal localization of the CPC, indicating an intimate linkage between Aurora B and Haspin functions in mitosis. We propose that Aurora B activity triggers a CPC-Haspin-H3T3ph feedback loop that promotes generation of H3T3ph on chromatin. We also provide evidence that the Bub1-Shugoshin-CPC pathway supplies a signal that boosts the CPC-Haspin-H3T3ph feedback loop specifically at centromeres to produce the well-known accumulation of the CPC in these regions.

Haspin inhibitors reveal that Aurora B at centromeres is required for metaphase chromosome alignment and spindle checkpoint signaling.

Human SWI/SNF (hSWI/SNF) is an evolutionarily conserved ATP-dependent chromatin remodeling complex required for transcriptional regulation and cell cycle control. The regulatory functions of hSWI/SNF are correlated with its ability to create a stable, altered form of chromatin that constrains fewer negative supercoils than normal. Our current studies indicate that this change in supercoiling is due to the conversion of up to one-half of the nucleosomes on polynucleosomal arrays into asymmetric structures, termed "altosomes," each composed of two histone octamers and bearing an asymmetrically located region of nuclease-accessible DNA. Altosomes can be formed on chromatin containing the abundant mammalian linker histone H1 and have a unique micrococcal nuclease digestion footprint that allows their position and abundance on any DNA sequence to be measured. Over time, altosomes spontaneously revert to structurally normal but improperly positioned nucleosomes, suggesting a novel mechanism for transcriptional attenuation as well as transcriptional memory following hSWI/SNF action.Human SWI/SNF (hSWI/SNF) is a chromatin remodeling complex that has essential functions in gene regulation, hormonal signaling, and cell cycle control (for recent reviews, see references 25, 34, and 35). It is the human member of the SWI/SNF family of remodeling complexes, which is remarkably conserved from Saccharomyces cerevisiae to humans. hSWI/SNF functions as a transcriptional coactivator when recruited to target genes through interaction with steroid receptors, as well as MyoD, ␤-catenin, Sp1, p53, and others. It also acts as a corepressor through interactions with Rb, REST, and prohibitin. hSWI/SNF function is correlated with its in vitro chromatin remodeling activities, including the translational repositioning of normal nucleosomes (3,38,41) and the generation of structurally altered products from mononucleosomes (11,22,26,38,40). hSWI/SNF also generates some form of stable, structurally altered product on polynucleosomes, as evidenced by changes in the supercoiling of circular plasmid chromatin in vivo and in vitro. The left-handed wrapping of DNA around the histone octamer results in one negative supercoil per nucleosome. When treated with hSWI/SNF in vitro, that number is decreased to about half (22,26). This change is stable but reverts back to normal on the timescale of several hours (15, 42). Critically, reversion occurs even in the presence of a vast excess of competitor DNA which would sequester any histones released by remodeling, indicating that the change in supercoiling is not due to histone loss but is instead due to some as yet unknown change in nucleosomal structure. A regulatory function for this altered structure is indicated by studies showing a correlation between SWI/SNF-dependent loss of supercoiling and transcription of genes on episomal plasmid DNAs (e.g., see references 24, 30, and 39). In addition, direct recruitment to GAL4 sites of the GAL4 DNA binding domain fused to the major hSWI/SNF catalytic ATPase, B...