SummaryKinetochores, multisubunit protein assemblies, connect chromosomes to spindle microtubules to promote chromosome segregation. The 10-subunit KMN assembly (comprising KNL1, MIS12, and NDC80 complexes, designated KNL1C, MIS12C, and NDC80C) binds microtubules and regulates mitotic checkpoint function through NDC80C and KNL1C, respectively. MIS12C, on the other hand, connects the KMN to the chromosome-proximal domain of the kinetochore through a direct interaction with CENP-C. The structural basis for this crucial bridging function of MIS12C is unknown. Here, we report crystal structures of human MIS12C associated with a fragment of CENP-C and unveil the role of Aurora B kinase in the regulation of this interaction. The structure of MIS12:CENP-C complements previously determined high-resolution structures of functional regions of NDC80C and KNL1C and allows us to build a near-complete structural model of the KMN assembly. Our work illuminates the structural organization of essential chromosome segregation machinery that is conserved in most eukaryotes.

Chromosomes are carriers of genetic material and their accurate transfer from a mother cell to its two daughters during cell division is of paramount importance for life. Kinetochores are crucial for this process, as they connect chromosomes with microtubules in the mitotic spindle. Kinetochores are multi-subunit complexes that assemble on specialized chromatin domains, the centromeres, that are able to enrich nucleosomes containing the histone H3 variant centromeric protein A (CENP-A). A group of several additional CENPs, collectively known as constitutive centromere associated network (CCAN), establish the inner kinetochore, whereas a ten-subunit assembly known as the KMN network creates a microtubule-binding site in the outer kinetochore. Interactions between CENP-A and two CCAN subunits, CENP-C and CENP-N, have been previously described, but a comprehensive understanding of CCAN organization and of how it contributes to the selective recognition of CENP-A has been missing. Here we use biochemical reconstitution to unveil fundamental principles of kinetochore organization and function. We show that cooperative interactions of a seven-subunit CCAN subcomplex, the CHIKMLN complex, determine binding selectivity for CENP-A over H3-nucleosomes. The CENP-A:CHIKMLN complex binds directly to the KMN network, resulting in a 21-subunit complex that forms a minimal high-affinity linkage between CENP-A nucleosomes and microtubules in vitro. This structural module is related to fungal point kinetochores, which bind a single microtubule. Its convolution with multiple CENP-A proteins may give rise to the regional kinetochores of higher eukaryotes, which bind multiple microtubules. Biochemical reconstitution paves the way for mechanistic and quantitative analyses of kinetochores.

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Faithful chromosome segregation is mandatory for cell and organismal viability. Kinetochores, large protein assemblies embedded in centromeric chromatin, establish a mechanical link between chromosomes and spindle microtubules. The KMN network, a conserved 10-subunit kinetochore complex, harbors the microtubule-binding interface. RWD domains in the KMN subunits Spc24 and Spc25 mediate kinetochore targeting of the microtubule-binding subunits by interacting with the Mis12 complex, a KMN subcomplex that tethers directly onto the underlying chromatin layer. Here, we show that Knl1, a KMN subunit involved in mitotic checkpoint signaling, also contains RWD domains that bind the Mis12 complex and that mediate kinetochore targeting of Knl1. By reporting the first 3D electron microscopy structure of the KMN network, we provide a comprehensive framework to interpret how interactions of RWD-containing proteins with the Mis12 complex shape KMN network topology. Our observations unveil a regular pattern in the construction of the outer kinetochore.

The apoptotic protease-activating factor 1 (Apaf-1) relays the death signal in the mitochondrial pathway of apoptosis. Apaf-1 oligomerizes on binding of mitochondrially released cytochrome c into the heptameric apoptosome complex to ignite the downstream cascade of caspases. Here, we present the 3.0 Å crystal structure of full-length murine Apaf-1 in the absence of cytochrome c. The structure shows how the mammalian death switch is kept in its "off" position. By comparing the off state with a recent cryo-electron microscopy derived model of Apaf-1 in its apoptosomal conformation, we depict the molecular events that transform Apaf-1 from autoinhibited monomer to a building block of the caspase-activating apoptosome. Moreover, we have solved the crystal structure of the R265S mutant of full-length murine Apaf-1 in the absence of cytochrome c to 3.55 Å resolution and we show that proper function of Apaf-1 relies on R265 in the vicinity of the bound nucleotide.

The Rod–Zw10–Zwilch (RZZ) complex assembles as a fibrous corona on kinetochores before microtubule attachment during mitotic spindle formation. Mosalaganti et al. provide new structural insight into the Spindly–RZZ complex that suggests that it resembles a dynein adaptor–cargo pair in the kinetochore corona.

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The solution structure of the Ras-binding domain (RBD) of Ral guanine-nucleotide exchange factor RalGEF was solved by NMR spectroscopy. The overall structure is similar to that of Raf-RBD, another effector of Ras, although the sequence identity is only 13%. 15N chemical shifts changes in the complex of RalGEF-RBD with Ras indicate an interaction similar to the intermolecular beta-sheet observed for the complex between Ras and Raf-RBD.