Electroporated recombinant proteins as tools for in vivo functional complementation, imaging and chemical biology

Delivery of native or chemically modified recombinant proteins into mammalian cells shows promise for functional investigations and various technological applications, but concerns that sub-cellular localization and functional integrity of delivered proteins may be affected remain high. Here, we surveyed batch electroporation as a delivery tool for single polypeptides and multi-subunit protein assemblies of the kinetochore, a spatially confined and well-studied subcellular structure. After electroporation into human cells, recombinant fluorescent Ndc80 and Mis12 multi-subunit complexes exhibited native localization, physically interacted with endogenous binding partners, and functionally complemented depleted endogenous counterparts to promote mitotic checkpoint signaling and chromosome segregation. Farnesylation is required for kinetochore localization of the Dynein adaptor Spindly. In cells with chronically inhibited farnesyl transferase activity, in vitro farnesylation and electroporation of recombinant Spindly faithfully resulted in robust kinetochore localization. Our data show that electroporation is well-suited to deliver synthetic and chemically modified versions of functional proteins, and, therefore, constitutes a promising tool for applications in chemical and synthetic biology.

The kinetochore proteins CENP-E and CENP-F directly and specifically interact with distinct BUB mitotic checkpoint Ser/Thr kinases

The segregation of chromosomes during cell division relies on the function of the kinetochores, protein complexes that physically connect chromosomes with microtubules of the spindle. The metazoan proteins, centromere protein E (CENP-E) and CENP-F, are components of a fibrous layer of mitotic kinetochores named the corona. Several of their features suggest that CENP-E and CENP-F are paralogs: they are very large (comprising ∼2700 and 3200 residues, respectively), contain abundant predicted coiled-coil structures, are C-terminally prenylated, and are endowed with microtubule-binding sites at their termini. Moreover, CENP-E contains an ATP-hydrolyzing motor domain that promotes microtubule plus end–directed motion. Here, we show that both CENP-E and CENP-F are recruited to mitotic kinetochores independently of the main corona constituent, the Rod/Zwilch/ZW10 (RZZ) complex. We identified specific interactions of CENP-F and CENP-E with budding uninhibited by benzimidazole 1 (BUB1) and BUB1-related (BUBR1) mitotic checkpoint Ser/Thr kinases, respectively, paralogous proteins involved in mitotic checkpoint control and chromosome alignment. Whereas BUBR1 was dispensable for kinetochore localization of CENP-E, BUB1 was stringently required for CENP-F localization. Through biochemical reconstitution, we demonstrated that the CENP-E/BUBR1 and CENP-F/BUB1 interactions are direct and require similar determinants, a dimeric coiled-coil in CENP-E or CENP-F and a kinase domain in BUBR1 or BUB1. Our findings are consistent with the existence of structurally similar BUB1/CENP-F and BUBR1/CENP-E complexes, supporting the notion that CENP-E and CENP-F are evolutionarily related.

BubR1 Promotes Bub3-Dependent APC/C Inhibition during Spindle Assembly Checkpoint Signaling

The spindle assembly checkpoint (SAC) prevents premature sister chromatid separation during mitosis. Phosphorylation of unattached kinetochores by the Mps1 kinase promotes recruitment of SAC machinery that catalyzes assembly of the SAC effector mitotic checkpoint complex (MCC). The SAC protein Bub3 is a phospho-amino acid adaptor that forms structurally related stable complexes with functionally distinct paralogs named Bub1 and BubR1. A short motif (“loop”) of Bub1, but not the equivalent loop of BubR1, enhances binding of Bub3 to kinetochore phospho-targets. Here, we asked whether the BubR1 loop directs Bub3 to different phospho-targets. The BubR1 loop is essential for SAC function and cannot be removed or replaced with the Bub1 loop. BubR1 loop mutants bind Bub3 and are normally incorporated in MCC in vitro but have reduced ability to inhibit the MCC target anaphase-promoting complex (APC/C), suggesting that BubR1:Bub3 recognition and inhibition of APC/C requires phosphorylation. Thus, small sequence differences in Bub1 and BubR1 direct Bub3 to different phosphorylated targets in the SAC signaling cascade.

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The molecular basis of kinetochore recruitment of Bub1 and BubR1 is dissected

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Bub1 and BubR1 modulate the ability of Bub3 to recognize phosphorylated targets

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A newly identified BubR1 motif targets Bub3 to the anaphase-promoting complex

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The newly identified motif of BubR1 is required for checkpoint signaling

Structure of the MIS12 Complex and Molecular Basis of Its Interaction with CENP-C at Human Kinetochores

Kinetochores, 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.

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We report a crystal structure of human MIS12 complex, a crucial kinetochore component

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The structure reveals how the MIS12 complex binds its kinetochore receptor CENP-C

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We dissect how Aurora B kinase promotes the MIS12:CENP-C interaction

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A combination of diverse structural methods reveals outer kinetochore organization

A molecular basis for the differential roles of Bub1 and BubR1 in the spindle assembly checkpoint

The spindle assembly checkpoint (SAC) monitors and promotes kinetochore–microtubule attachment during mitosis. Bub1 and BubR1, SAC components, originated from duplication of an ancestor gene. Subsequent sub-functionalization established subordination: Bub1, recruited first to kinetochores, promotes successive BubR1 recruitment. Because both Bub1 and BubR1 hetero-dimerize with Bub3, a targeting adaptor for phosphorylated kinetochores, the molecular basis for such sub-functionalization is unclear. We demonstrate that Bub1, but not BubR1, enhances binding of Bub3 to phosphorylated kinetochores. Grafting a short motif of Bub1 onto BubR1 promotes Bub1-independent kinetochore recruitment of BubR1. This gain-of-function BubR1 mutant cannot sustain a functional checkpoint. We demonstrate that kinetochore localization of BubR1 relies on direct hetero-dimerization with Bub1 at a pseudo-symmetric interface. This pseudo-symmetric interaction underpins a template–copy relationship crucial for kinetochore–microtubule attachment and SAC signaling. Our results illustrate how gene duplication and sub-functionalization shape the workings of an essential molecular network.

DOI:http://dx.doi.org/10.7554/eLife.05269.001

The genetic material within our cells is arranged in structures called chromosomes. Before a cell divides it makes an accurate copy of all of its DNA. The genetic material then needs to be equally split so that both daughter cells have a complete set of chromosomes.

As the cell prepares to divide, each chromosome—consisting of two identical sister chromatids—lines up on a structure known as the spindle, which is made of filaments called microtubules. Cells have a sophisticated safety mechanism known as the spindle assembly checkpoint to ensure that chromosomes have time to correctly line up on the spindle before the cell can divide. Once this checkpoint is satisfied, the microtubules pull the sister chromatids apart so that each daughter cell receives one chromatid from each pair.

The microtubules attach to the chromosomes through a large protein complex known as the kinetochore that assembles on each sister chromatid. The spindle assembly checkpoint monitors the attachment of the kinetochores to the microtubules; and two proteins, called Bub1 and BubR1, play an essential role in this process. These proteins bind to another protein called Bub3 that is also part of the spindle assembly checkpoint. Although Bub1 and BubR1 are very similar, they do not appear to perform the same roles, but the precise molecular details of their differences remain unclear.

In this study, Overlack, Primorac et al. studied Bub1 and BubR1 in human cells. The experiments show that Bub1 can be recruited to kinetochores in the absence of BubR1, but BubR1 will only move to kinetochores when Bub1 is present. Furthermore, BubR1 needs to bind to Bub1 directly to move to the kinetochores. Overlack, Primorac et al. also identified a region in Bub1 that binds to Bub3, and which is considerably different in BubR1. When this region of Bub1 was grafted into BubR1, the resulting protein was able to bind kinetochores even in the absence of Bub1.

The genes that encode the Bub1 and BubR1 proteins originate from a single ancestor gene that was duplicated during evolution. Therefore, the findings of Overlack, Primorac et al. show how the duplication of a gene can be beneficial for cells by creating products that have different roles in cells.

DOI:http://dx.doi.org/10.7554/eLife.05269.002

When Mad met Bub

The faithful segregation of chromosomes into daughter cells is essential for cellular and organismal viability. Errors in this process cause aneuploidy, a hallmark of cancer and several congenital diseases. For proper separation, chromosomes attach to microtubules of the mitotic spindle via their kinetochores, large protein structures assembled on centromeric chromatin. Kinetochores are also crucial for a cell cycle feedback mechanism known as the spindle assembly checkpoint (SAC). The SAC forces cells to remain in mitosis until all chromosomes are properly attached to microtubules. At the beginning of mitosis, the SAC proteins--Mad1, Mad2, Bub1, Bub3, BubR1, Mps1, and Cdc20--are recruited to kinetochores in a hierarchical and interdependent fashion (Fig 1A). There they monitor, in ways that are not fully clarified, the formation of kinetochore-microtubule attachments. Two studies recently published in EMBO reports by the groups of Silke Hauf and Jakob Nilsson, and a recent study by London and Biggins in Genes & Development, shed new light on the conserved SAC protein Mad1.

Modular assembly of RWD domains on the Mis12 complex underlies outer kinetochore organization

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.