DomainFit: Identification of protein domains in cryo-EM maps at intermediate resolution using AlphaFold2-predicted models

Cryoelectron microscopy (cryo-EM) has revolutionized the structural determination of macromolecular complexes. With the paradigm shift to structure determination of highly complex endogenous macromolecular complexes ex vivo and in situ structural biology, there are an increasing number of structures of native complexes. These complexes often contain unidentified proteins, related to different cellular states or processes. Identifying proteins at resolutions lower than 4 Å remains challenging because side chains cannot be visualized reliably. Here, we present DomainFit, a program for semi-automated domain-level protein identification from cryo-EM maps, particularly at resolutions lower than 4 Å. By fitting domains from AlphaFold2-predicted models into cryo-EM maps, the program performs statistical analyses and attempts to identify the domains and protein candidates forming the density. Using DomainFit, we identified two microtubule inner proteins, one of which contains a CCDC81 domain and is exclusively localized in the proximal region of the doublet microtubule in Tetrahymena thermophila.

Effect of α-tubulin acetylation on the doublet microtubule structure

Acetylation of α-tubulin at the lysine 40 residue (αK40) by αTAT1/MEC-17 acetyltransferase modulates microtubule properties and occurs in most eukaryotic cells. Previous literatures suggest that acetylated microtubules are more stable and damage resistant. αK40 acetylation is the only known microtubule luminal post-translational modification site. The luminal location suggests that the modification tunes the lateral interaction of protofilaments inside the microtubule. In this study, we examined the effect of tubulin acetylation on the doublet microtubule (DMT) in the cilia of Tetrahymena thermophila using a combination of cryo-electron microscopy, molecular dynamics, and mass spectrometry. We found that αK40 acetylation exerts a small-scale effect on the DMT structure and stability by influencing the lateral rotational angle. In addition, comparative mass spectrometry revealed a link between αK40 acetylation and phosphorylation in cilia.

A conserved CENP-E region mediates BubR1-independent recruitment to the outer corona at mitotic onset

The outer corona plays an essential role at the onset of mitosis by expanding to maximize microtubule attachment to kinetochores.1,2 The low-density structure of the corona forms through the expansion of unattached kinetochores. It comprises the RZZ complex, the dynein adaptor Spindly, the plus-end directed microtubule motor centromere protein E (CENP-E), and the Mad1/Mad2 spindle-assembly checkpoint proteins.3,4,5,6,7,8,9,10 CENP-E specifically associates with unattached kinetochores to facilitate chromosome congression,11,12,13,14,15,16 interacting with BubR1 at the kinetochore through its C-terminal region (2091-2358).17,18,19,20,21 We recently showed that CENP-E recruitment to BubR1 at the kinetochores is both rapid and essential for correct chromosome alignment. However, CENP-E is also recruited to the outer corona by a second, slower pathway that is currently undefined.19 Here, we show that BubR1-independent localization of CENP-E is mediated by a conserved loop that is essential for outer-corona targeting. We provide a structural model of the entire CENP-E kinetochore-targeting domain combining X-ray crystallography and Alphafold2. We reveal that maximal recruitment of CENP-E to unattached kinetochores critically depends on BubR1 and the outer corona, including dynein. Ectopic expression of the CENP-E C-terminal domain recruits the RZZ complex, Mad1, and Spindly, and prevents kinetochore biorientation in cells. We propose that BubR1-recruited CENP-E, in addition to its essential role in chromosome alignment to the metaphase plate, contributes to the recruitment of outer corona proteins through interactions with the CENP-E corona-targeting domain to facilitate the rapid capture of microtubules for efficient chromosome alignment and mitotic progression.

DomainFit: Identification of Protein Domains in cryo-EM maps at Intermediate Resolution using AlphaFold2-predicted Models

Cryo-electron microscopy (cryo-EM) has revolutionized our understanding of macromolecular complexes, enabling high-resolution structure determination. With the paradigm shift to in situ structural biology recently driven by the ground-breaking development of cryo-focused ion beam milling and cryo-electron tomography, there are an increasing number of structures at sub-nanometer resolution of complexes solved directly within their cellular environment. These cellular complexes often contain unidentified proteins, related to different cellular states or processes. Identifying proteins at resolutions lower than 4 Å remains challenging because the side chains cannot be visualized reliably. Here, we present DomainFit, a program for automated domain-level protein identification from cryo-EM maps at resolutions lower than 4 Å. By fitting domains from artificial intelligence-predicted models such as AlphaFold2-predicted models into cryo-EM maps, the program performs statistical analyses and attempts to identify the proteins forming the density. Using DomainFit, we identified two microtubule inner proteins, one of them, a CCDC81 domain-containing protein, is exclusively localized in the proximal region of the doublet microtubule from the ciliate Tetrahymena thermophila. The flexibility and capability of DomainFit makes it a valuable tool for analyzing in situ structures.

CEP104/FAP256 and associated cap complex maintain stability of the ciliary tip

Cilia are essential organelles that protrude from the cell body. Cilia are made of a microtubule-based structure called the axoneme. In most types of cilia, the ciliary tip is distinct from the rest of the cilium. Here, we used cryo-electron tomography and subtomogram averaging to obtain the structure of the ciliary tip of the ciliate Tetrahymena thermophila. We show that the microtubules at the tip are highly crosslinked with each other and stabilized by luminal proteins, plugs, and cap proteins at the plus ends. In the tip region, the central pair lacks typical projections and twists significantly. By analyzing cells lacking a ciliary tip-enriched protein CEP104/FAP256 by cryo-electron tomography and proteomics, we discovered candidates for the central pair cap complex and explained the potential functions of CEP104/FAP256. These data provide new insights into the function of the ciliary tip and the mechanisms of ciliary assembly and length regulation.

Phosphorylation controls spatial and temporal activities of motor-PRC1 complexes to complete mitosis

During mitosis, spindle architecture alters as chromosomes segregate into daughter cells. The microtubule crosslinker protein regulator of cytokinesis 1 (PRC1) is essential for spindle stability, chromosome segregation and completion of cytokinesis, but how it recruits motors to the central spindle to coordinate the segregation of chromosomes is unknown. Here, we combine structural and cell biology approaches to show that the human CENP-E motor, which is essential for chromosome capture and alignment by microtubules, binds to PRC1 through a conserved hydrophobic motif. This binding mechanism is also used by Kinesin-4 Kif4A:PRC1. Using in vitro reconstitution, we demonstrate that CENP-E slides antiparallel PRC1-crosslinked microtubules. We find that the regulation of CENP-E -PRC1 interaction is spatially and temporally coupled with relocalization to overlapping microtubules in anaphase. Finally, we demonstrate that the PRC1-microtubule motor interaction is essential in anaphase to control chromosome partitioning, retain central spindle integrity and ensure cytokinesis. Taken together our findings reveal the molecular basis for the cell cycle regulation of motor-PRC1 complexes to couple chromosome segregation and cytokinesis.

Integrated modeling of the Nexin-dynein regulatory complex reveals its regulatory mechanism

Cilia are hairlike protrusions that project from the surface of eukaryotic cells and play key roles in cell signaling and motility. Ciliary motility is regulated by the conserved nexin-dynein regulatory complex (N-DRC), which links adjacent doublet microtubules and regulates and coordinates the activity of outer doublet complexes. Despite its critical role in cilia motility, the assembly and molecular basis of the regulatory mechanism are poorly understood. Here, using cryo-electron microscopy in conjunction with biochemical cross-linking and integrative modeling, we localize 12 DRC subunits in the N-DRC structure of Tetrahymena thermophila. We also find that the CCDC96/113 complex is in close contact with the DRC9/10 in the linker region. In addition, we reveal that the N-DRC is associated with a network of coiled-coil proteins that most likely mediates N-DRC regulatory activity.

Integrated modeling of the Nexin-dynein regulatory complex reveals its regulatory mechanism

Cilia are hairlike protrusions that project from the surface of eukaryotic cells and play key roles in cell signaling and motility. Ciliary motility is regulated by the conserved nexin-dynein regulatory complex (N-DRC), which links adjacent doublet microtubules and regulates and coordinates the activity of outer doublet complexes. Despite its critical role in cilia motility, the assembly and molecular basis of the regulatory mechanism are poorly understood. Here, utilizing cryo-electron microscopy in conjunction with biochemical cross-linking and integrative modeling, we localized 12 DRC subunits in the N-DRC structure of Tetrahymena thermophila . We also found that the CCDC96/113 complex is in close contact with the N-DRC. In addition, we revealed that the N-DRC is associated with a network of coiled-coil proteins that most likely mediates N-DRC regulatory activity.

Native doublet microtubules from Tetrahymena thermophila reveal the importance of outer junction proteins

Cilia are ubiquitous eukaryotic organelles responsible for cellular motility and sensory functions. The ciliary axoneme is a microtubule-based cytoskeleton consisting of two central singlets and nine outer doublet microtubules. Cryo-electron microscopy-based studies have revealed a complex network inside the lumen of both tubules composed of microtubule-inner proteins (MIPs). However, the functions of most MIPs remain unknown. Here, we present single-particle cryo-EM-based analyses of the Tetrahymena thermophila native doublet microtubule and identify 42 MIPs. These data shed light on the evolutionarily conserved and diversified roles of MIPs. In addition, we identified MIPs potentially responsible for the assembly and stability of the doublet outer junction. Knockout of the evolutionarily conserved outer junction component CFAP77 moderately diminishes Tetrahymena swimming speed and beat frequency, indicating the important role of CFAP77 and outer junction stability in cilia beating generation and/or regulation.

Molecular architecture of the ciliary tip revealed by cryo-electron tomography

Cilia are essential organelles that protrude from the cell body. Cilia are made of a microtubule-based structure called the axoneme. In most types of cilia, the ciliary tip is distinct from the rest of the cilium. Here, we used cryo-electron tomography and subtomogram averaging to obtain the structure of the ciliary tip of the ciliate Tetrahymena thermophila. We show the microtubules in the tip are highly cross-linked with each other and stabilised by luminal proteins, plugs and cap proteins at the plus ends. In the tip region, the central pair lacks the typical projections and twists significantly. By analysing cells lacking a ciliary tip-enriched protein CEP104/FAP256 by cryo-electron tomography and proteomics, we discovered candidates for the central pair cap complex and explain potential functions of CEP104/FAP256. These data provide new insights into the function of the ciliary tip and inform about the mechanisms of ciliary assembly and length regulation.

Reconstitution of an active human CENP-E motor

CENP-E is a large kinesin motor protein which plays pivotal roles in mitosis by facilitating chromosome capture and alignment, and promoting microtubule flux in the spindle. So far, it has not been possible to obtain active human CENP-E to study its molecular properties. Xenopus CENP-E motor has been characterized in vitro and is used as a model motor; however, its protein sequence differs significantly from human CENP-E. Here, we characterize human CENP-E motility in vitro. Full-length CENP-E exhibits an increase in run length and longer residency times on microtubules when compared to CENP-E motor truncations, indicating that the C-terminal microtubule-binding site enhances the processivity when the full-length motor is active. In contrast with constitutively active human CENP-E truncations, full-length human CENP-E has a reduced microtubule landing rate in vitro, suggesting that the non-motor coiled-coil regions self-regulate motor activity. Together, we demonstrate that human CENP-E is a processive motor, providing a useful tool to study the mechanistic basis for how human CENP-E drives chromosome congression and spindle organization during human cell division.

The C-terminal helix of BubR1 is essential for CENP-E-dependent chromosome alignment

During cell division, misaligned chromosomes are captured and aligned by motors before their segregation. The CENP-E motor is recruited to polar unattached kinetochores to facilitate chromosome alignment. The spindle checkpoint protein BubR1 (also known as BUB1B) has been reported as a CENP-E interacting partner, but the extent to which BubR1 contributes to CENP-E localization at kinetochores has remained controversial. Here we define the molecular determinants that specify the interaction between BubR1 and CENP-E. The basic C-terminal helix of BubR1 is necessary but not sufficient for CENP-E interaction, and a minimal key acidic patch on the kinetochore-targeting domain of CENP-E is also essential. We then demonstrate that BubR1 is required for the recruitment of CENP-E to kinetochores to facilitate chromosome alignment. This BubR1-CENP-E axis is critical for alignment of chromosomes that have failed to congress through other pathways and recapitulates the major known function of CENP-E. Overall, our studies define the molecular basis and the function for CENP-E recruitment to BubR1 at kinetochores during mammalian mitosis.This article has an associated First Person interview with the first author of the paper.

Molecular architecture of the Dam1 complex-microtubule interaction

Mitosis is a highly regulated process that allows the equal distribution of the genetic material to the daughter cells. Chromosome segregation requires the formation of a bipolar mitotic spindle and assembly of a multi-protein structure termed the kinetochore to mediate attachments between condensed chromosomes and spindle microtubules. In budding yeast, a single microtubule attaches to each kinetochore, necessitating robustness and processivity of this kinetochore-microtubule attachment. The yeast kinetochore-localized Dam1 complex forms a direct interaction with the spindle microtubule. In vitro, the Dam1 complex assembles as a ring around microtubules and couples microtubule depolymerization with cargo movement. However, the subunit organization within the Dam1 complex, its higher-order oligomerization and how it interacts with microtubules remain under debate. Here, we used chemical cross-linking and mass spectrometry to define the architecture and subunit organization of the Dam1 complex. This work reveals that both the C termini of Duo1 and Dam1 subunits interact with the microtubule and are critical for microtubule binding of the Dam1 complex, placing Duo1 and Dam1 on the inside of the ring structure. Integrating this information with available structural data, we provide a coherent model for how the Dam1 complex self-assembles around microtubules.