Research Article

Structure of nucleosome-bound human BAF complex

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Science  21 Feb 2020:
Vol. 367, Issue 6480, pp. 875-881
DOI: 10.1126/science.aaz9761

Architecture of human BAF complex

The SWI/SNF family chromatin remodelers regulate chromatin and transcription. The protein complexes BAF and PBAF are mammalian SWI/SNF remodelers that play essential functions in diverse developmental and physiological processes. He et al. determined the structure of the human BAF complex, which contains three modules that bind the nucleosome on the top, bottom, and side, making this nucleosome-recognition pattern distinct from other chromatin remodelers. Mutations in BAF that are frequently associated with human cancer cluster into a nucleosome-interacting region. This structure provides a framework for understanding the BAF-mediated chromatin remodeling mechanism and its dysregulation in cancer.

Science, this issue p. 875


Mammalian SWI/SNF family chromatin remodelers, BRG1/BRM-associated factor (BAF) and polybromo-associated BAF (PBAF), regulate chromatin structure and transcription, and their mutations are linked to cancers. The 3.7-angstrom-resolution cryo–electron microscopy structure of human BAF bound to the nucleosome reveals that the nucleosome is sandwiched by the base and the adenosine triphosphatase (ATPase) modules, which are bridged by the actin-related protein (ARP) module. The ATPase motor is positioned proximal to nucleosomal DNA and, upon ATP hydrolysis, engages with and pumps DNA along the nucleosome. The C-terminal α helix of SMARCB1, enriched in positively charged residues frequently mutated in cancers, mediates interactions with an acidic patch of the nucleosome. AT-rich interactive domain-containing protein 1A (ARID1A) and the SWI/SNF complex subunit SMARCC serve as a structural core and scaffold in the base module organization, respectively. Our study provides structural insights into subunit organization and nucleosome recognition of human BAF complex.

The adenosine triphosphate (ATP)–dependent chromatin remodeling complexes (also known as chromatin remodelers) regulate the chromatin packing state by sliding, ejecting, and restructuring the nucleosome to enable dynamic regulation of chromatin structure (1, 2). As prototype chromatin remodelers, the SWI/SNF complexes demonstrate nucleosome sliding activity and distinctive ejection activity, by which they create nucleosome-depleted regions (NDRs) that are essential for transcriptional regulation (310). Mammalian SWI/SNF (mSWI/SNF) complexes, BRG1/BRM-associated factor (BAF) and polybromo-associated BAF (PBAF), consist of up to 15 subunits encoded by more than 29 genes, generating more than 1400 possible complexes (6, 7, 11, 12). Up to 20% of malignancies contain mutations of BAF and/or PBAF subunits, making these complexes among the most frequently dysregulated targets in human cancer (4, 7, 13).

Although the compositions and subunit functions of SWI/SNF complexes have been extensively studied (8, 11, 1419), the structural studies of SWI/SNF complexes have been limited to the low-resolution electron microscopy (EM) structures of yeast SWI/SNF complexes (2023) and structures of isolated domains (15, 2427). A recent study reported an ~7-Å-resolution structure of RSC (yeast homolog of PBAF) bound to the nucleosome with the substrate recruitment module refined to 3.4-Å resolution (28). However, the molecular mechanisms of subunit organization and nucleosome recognition of mammalian BAF complex remain largely unknown.

Structure determination

We reconstituted human BAF complex consisting of the catalytic subunit SMARCA4 (BRG1) and nine auxiliary subunits (Fig. 1A, figs. S1 and S2, and table S1). The purified BAF complexes exhibited nucleosome sliding activities (fig. S1, D and H to J) and were complexed with nucleosome core particle (NCP) in the absence of ATP and adenosine diphosphate (ADP) (fig. S1D). The cryo-EM structure of the BAF-NCP complex (~1.2 MDa) was determined at an overall resolution of 3.7 Å, with the map of the base module locally refined to 3.0-Å resolution (Figs. 1, B to F, and 2A; figs. S3 and S4; table S2; and movies S1 to S5). The structural models were built by fitting available structures into the cryo-EM maps (15, 24, 26, 27, 29) followed by manual model building aided by cross-linking mass spectrometry (XL-MS) (fig. S2, A and B, and data S1 and S2). The 3.7-Å-resolution map and corresponding model were used hereafter unless otherwise specified.

Fig. 1 Cryo-EM structure of human BAF bound to NCP.

(A) Schematic architecture of BAF complex organization and domain structure of BAF subunits. Color scheme for BAF subunits is indicated in (G) and used throughout all figures. (B to D) The 3.7-Å-resolution cryo-EM map of BAF-NCP in the absence of ADP in three different views. The map at a higher threshold level [(B), right panel] reveals more details, with the ATPase module fading out. The map at lower threshold level [(B), left panel] reveals density (likely derived from SnAC) packing across the top surface of nucleosome. The intermodular contacts and BAF-NCP contacts are indicated with white circles. (E and F) Two different views of the cryo-EM map of the nucleosome showing detached and disordered DNA at entry (E) and exit (F) sites. The positions of nucleosomal DNA are labeled with SHL numbers. (G) Cartoon model of BAF-NCP structure shown in two different views. RPT, repeat domain; Req, Requiem domain; SWIRM, SWI3, RSC8, and MOIRA; SANT, Swi3, Ada2, N-Cor, and TFIIIB; ARM, armadillo repeats; CC, coiled coil; HSA, helicase-SANT–associated; ARP, actin-related protein; BR, bromodomain; WH, winged helix; SWIB, SWI/SNF complex B/MDM2.

Fig. 2 Structure of the base module.

(A) The cryo-EM map of the base module locally refined to 3.0-Å resolution. The submodules and elements involved in complex assembly are indicated. (B) Cartoon model of the base module in two different views. Regions that are involved in nucleosome recognition and complex formation are indicated. CC-C2a* and CC-C2b* represent the possibility of two CCs of SMARCC2 because of a disconnected cryo-EM map.

BAF sandwiches the nucleosome

The human BAF complex sandwiches the nucleosome, and its nucleosome-binding manner is distinct from that of other representative chromatin remodelers, including Chd1, SWR1, and INO80 (3032) (Fig. 1, B to D, and fig. S5). These remodelers primarily bind nucleosomal DNA and/or histone tails but have fewer contacts with core histones. The sandwiched nucleosome binding of BAF may provide a structural basis to support nucleosome ejection activity (discussed below). The nucleosomal DNA tends to be detached or disordered at both entry and exit sites (Fig. 1, E and F, and movie S5), which may allow the DNA translocation to occur more efficiently owing to fewer restraints.

The BAF complex consists of three modules: the adenosine triphosphatase (ATPase) module, the actin-related protein (ARP) module, and the base module (Fig. 1A). A large portion of the catalytic subunit SMARCA4 (residues 521 to 1647) forms the ATPase module, which grasps the nucleosome with the ATPase motor partially wrapping around the nucleosomal DNA (Fig. 1, B to D). Within the ARP module, the helicase-SANT–associated region (HSA; residues 446 to 520) of SMARCA4 binds the heterodimer formed by ACTL6A (BAF53A) and ACTB (β-actin) (Fig. 1, A and B). The pre-HSA (residues 350 to 445) of SMARCA4 is anchored into the base module, in which the SMARCB1 (BAF47, hSNF5, or INI1) packs against the bottom surface of the nucleosome (Fig. 1, A and B).

The ARP module bridges the ATPase and base modules

The ARP module is formed by the ACTL6A-ACTB heterodimer and the long α helix of the HSA of SMARCA4 (Fig. 1G), revealing a fold similar to the yeast HSASnf2-arp7-arp9-Rtt102 structure (27). The ARP module has no direct contact with the nucleosome, but it associates with and bridges the ATPase and base modules (Fig. 1, B, C, and G). A cryo-EM map reveals considerable contacts between lobe 1 of the SMARCA4 ATPase domain and subdomain 4 (residues 266 to 305) of ACTL6A (Fig. 1, B and G). Moreover, the pre-HSA in the base module and post-HSA in the ATPase module are directly connected by the HSA helix. Thus, the ARP module maintains a rigid conformation of the HSA helix and likely couples the motions of the ATPase and base modules during chromatin remodeling. This observation is consistent with the essential role of the ARP module both in “coupling” the DNA translocation and ATP hydrolysis and in the assembly of functional SWI/SNF complexes (8, 18, 27).

The ATPase motor engages with nucleosomal DNA

The cryo-EM density of the ATPase motor is relatively weak and reveals a tilted and open conformation (angle of ~90° between the two ATPase lobes), indicating that the structure represents a pre-engaged conformation (fig. S6), which is consistent with the lack of ATP and ADP in the BAF-NCP structure. The ATPase motor is positioned at nucleosomal DNA near superhelical location (SHL) 2.5, and the positioning is likely guided by the HSA-associated ARP module and ATPase-associated Snf2 ATP coupling (SnAC) domain. We observed weak cryo-EM density stretching across the top surface of the nucleosome (Fig. 1D and fig. S6A). This region is likely derived from the SnAC and/or bromodomain, which have been shown to bind DNA and/or histone (25, 33, 34).

The structure of BAF-NCP in the presence of ADP was refined to 10.3-Å resolution, showing that the ATPase motor adopts a relatively closed conformation (~70°) and has tight contacts to nucleosomal DNA around SHL 2 (fig. S6B and movie S6). Thus, ATP or ADP would promote conformational transition of the BAF complex from a pre-engaged to an engaged state, with the ATPase engaging with nucleosomal DNA and, upon ATP hydrolysis, pumping DNA toward the nucleosome dyad and generating DNA translocation.

The organization of the base module

The pre-HSA of SMARCA4 is anchored into the base module, which consists of seven additional auxiliary subunits: two BAF-specific subunits, ARID1A (BAF250A) and DPF2 (BAF45D); and five BAF/PBAF-shared subunits, SMARCB1, SMARCD1 (BAF60A), SMARCE1 (BAF57), and two copies of SMARCC (8, 11, 1519) (Figs. 1 and 2). These auxiliary subunits exist exclusively in the base module and account for ~80% of the total molecular mass of the BAF complex (fig. S4).

The base module reveals a compact fold and can be divided into five closely associated submodules: the head, thumb, palm, bridge, and fingers (Fig. 2). The head and bridge bind the nucleosome and the ARP module, respectively, generating intermodular contacts (Fig. 1G). The thumb is formed by a SANT domain of SMARCC, the pre-HSA of SMARCA4, and the C-terminal helices of SMARCD1 (Fig. 2). The fingers submodule reveals a characteristic Y-shaped five-helix bundle formed by coiled-coil (CC) domains of SMARCD1, SMARCE1, and two SMARCC subunits. The palm connects pre-HSA, SMARCD1, SMARCE1, and SMARCC, which merge at a four-way junction.

Nucleosome recognition by SMARCB1 in the head submodule

The head directly binds the histone octamer on the bottom (Fig. 1, B and C, and fig. S7A). It consists of two repeat (RPT) domains and a C-terminal α (αC) helix of SMARCB1, the requiem (Req) domain of DPF2, two SWIRM domains of SMARCCs, and an insert derived from ARID1A (ARID1A-insert) (Fig. 3A and fig. S7B). The two SWIRM domains bind the RPT1 and RPT2 domains, respectively. The two RPT-SWIRM complexes adopt a similar fold and asymmetrically bind each other, with the intermolecular interactions buttressed by the ARID1A-insert (residues 1802 to 1862) and Req domain (residues 13 to 82) of DPF2 (Fig. 3, A and B). The head merges with the bridge and thumb at a three-way junction, and the interactions are mediated by the interwoven loops from DPF2, ARID1A, and two SMARCCs (Fig. 3A). The Req domain of DPF2 is almost identical to that of DPF1 and DPF3 in primary sequence, indicating a similar role of DPF1, DPF2, and DPF3 in BAF complex organization (fig. S2D).

Fig. 3 Structure of the head submodule and role of SMARCB1 in nucleosome recognition.

(A) Close-up view of the head submodule and its interactions with the bridge and thumb submodules. The sandwiched loops located in a three-way junction are indicated with a dashed blue circle. (B) Close-up view of interactions between the Req domain of DPF2 shown in cartoon and RTP2-SWIRMb shown in surface representation (red, negative charge; blue, positive charge). (C) Close-up view of the contacts between SMARCB1 and the bottom surface of the nucleosome. Four histone-contacting arginine residues on the αC helix are shown in stick representation. Single-letter abbreviations for the amino acid residues are as follows: A, Ala; C, Cys; D, Asp; E, Glu; F, Phe; G, Gly; H, His; I, Ile; K, Lys; L, Leu; M, Met; N, Asn; P, Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; W, Trp; and Y, Tyr. (D) Basic residues of the αC helix pack against the acidic surface of the histone octamer. The positively charged residues are shown as sticks. (E) Conservation of the αC helix of SMARCB1. The invariant and highly conserved residues are highlighted with dark-green and green backgrounds, respectively. (F) The cryo-EM map around the αC helix is shown in mesh in two views. The side chains of the abovementioned residues are well covered by the map.

SMARCB1 is required for structural integrity and function of the SWI/SNF complex (16, 17, 35), and inactivation of SMARCB1 was reported in almost all malignant rhabdoid tumors (14, 3638). The αC helix, which is enriched in residues that are frequently mutated in human cancers, packs against the bottom H2A-H2B heterodimer and serves as a hinge to connect the base module and nucleosome (Fig. 3C and fig. S7A). The four most frequently mutated arginine residues (R370, R373, R374, and R377) together form a positively charged cluster and pack against an acidic patch on the bottom of the nucleosome (Fig. 3D). The four arginine residues are invariant across species, from yeast to humans (Fig. 3E), and were clearly visualized in the cryo-EM map (Fig. 3F). This observation highlights the importance of the αC helix of SMARCB1 in binding of the nucleosome and is consistent with recent studies (14, 28). Although the αC helix of Sfh1 (yeast homolog of SMARCB1) is disordered in the yeast RSC-NCP complex structure, the deletion of this helix impairs nucleosome ejection activity (28).

The RPT1 domain of SMARCB1 is in close proximity to the α2 helix of histone H2A and likely causes steric clash between the histone octamer and DNA (Figs. 1, B and G, and 3C; and fig. S5A), suggesting that RPT1 serves as a “wedge” that favors DNA detachment around the exit site. The N-terminal winged helix (WH) domain of SMARCB1 binds the ARM domain of ARID1A and is located more than 40 Å away from nucleosomal DNA (Fig. 1, C and G), suggesting a role independent of its previously reported DNA binding ability (26). The WH domain is in close proximity to the ARP module, suggesting a role in regulating ARP–base intermodular interactions (Fig. 1, B and C).

ARID1A serves as a rigid core to stabilize the base module

ARID1A is the largest subunit of the BAF complex. A large portion of ARID1A, including the characteristic AT-rich interacting domain (ARID), was invisible in the complex structure, possibly owing to intrinsic flexibility (fig. S2D). The modeled C-terminal ARM domain consists of seven ARM repeats arranged in a superhelical conformation and serves as a rigid core to bind SMARCA4 and all other base subunits (Fig. 4A). A zinc finger is formed with a zinc atom coordinated with residues H2019 and H2021 on ARM3 and residues H2090 and C2094 on ARM4 (Fig. 4B). The zinc finger stabilizes two ARM repeats and the associated loop regions. The ARM domain is located in the center of the base module, with the bottom associating with the fingers, one end joining with the head and thumb, and the other end capped by the SWIB domain of SMARCD1. The N-terminal helical turns of HSA and the preceding loop pack against the concave surface of the ARM domain. The surface residues of ARM contacting SMARCA4 and SMARCD1 are highly conserved, suggesting evolutionarily conserved functions in stabilizing the base module.

Fig. 4 Subunit organization in the base module.

(A) Cartoon model of interactions between ARID1A and other subunits is shown in two views. (Left middle panel) The surface of the ARM domain is shown and colored according to the conservation scores. (Right middle panel) Seven ARM repeats are shown, with front helices colored in yellow, back helices colored in green, and ridge helices and loops colored in blue. (B) Close-up view of the zinc finger connecting ARM3 and ARM4, with zinc-coordinating residues shown in sticks and zinc atom shown as a gray ball. The location of the zinc finger is indicated with a blue rectangle in the upper panel of (A). (C) Cartoon of the base module with ARID1A and SMARCA4 omitted. The lack of ARID1A would lead to a clash of the base module, although other subunits remain associated by the scaffold subunits, SMARCC1 and SMARCC2. The three-pronged yellow and blue shape indicates a three-way junction of the head, thumb, and bridge. (D) In vitro nucleosome sliding assay performed using purified BAF∆ARID1A complex and increasing amounts of purified ARID1A. Note that BAF∆ARID1A demonstrates nucleosome sliding activity in higher protein concentration (fig. S8D). 45N45, a center-positioned nucleosome with two 45–base pair flanking DNA fragments.

We found that, in a manner consistent with the central role of the ARM domain in base module organization, ARID1A bound to other base subunits and the purified base subcomplex (fig. S8, A to C). ARID1A is one of the most frequently mutated mSWI/SNF subunits in human cancer, and frameshift mutations occur near the C terminus (11, 39), supporting the importance of the C-terminal ARM domain in BAF structure and function. The structure showed that the overall fold of the BAF complex would not be properly maintained in the absence of ARID1A (Fig. 4C), although other BAF subunits would remain associated with SMARCC1 and SMARCC2 because the BAF∆ARID1A complex survived ion-exchange chromatography (fig. S1C). The sliding activity of BAF∆ARID1A was considerably enhanced by the addition of larger amounts of purified ARID1A (Fig. 4D and fig. S8D), indicating that ARID1A is required for efficient nucleosome sliding activity.

ARID1A and ARID1B share highly similar primary sequences and are mutually exclusive in the BAF complex, suggesting that ARID1B-containing BAF is assembled in a similar manner (fig. S2D). As a PBAF-specific subunit, ARID2 is not a paralog of ARID1A or ARID1B and shows a distinct domain architecture. However, the N-terminal helical region (residues 150 to 470) of ARID2 is predicted to form a seven-repeat ARM domain (40). Thus, the N-terminal ARM domain of ARID2 and the C-terminal ARM domain of ARID1A may play a similar role in organizing PABF and BAF, respectively.

SMARCCs compose base module scaffold, and SMARCD1 and SMARCE1 facilitate organization

As the scaffold of the base module, the two SMARCC subunits bind all other base subunits and thread through the head, thumb, palm, and fingers submodules (Fig. 4C). The BAF complexes containing SMARCC1 and SMARCC2 (BAF-CC1/CC2) and two SMARCC2 subunits (BAF-CC2/CC2) generated almost identical cryo-EM maps (fig. S3). SMARCC1 and SMARCC2 behave similarly in binding other BAF subunits and subcomplexes (fig. S9, D to I, and supplementary text). This observation is consistent with a previous study showing that SMARCC1 and SMARCC2 are functionally similar and play critical roles in the early stage of mSWI/SNF complex assembly (11).

SMARCD1 adopts an elongated conformation and runs alongside the two SMARCC subunits (Fig. 4C). The SWIB domain adopts a compact globular fold and binds the ARM7 of ARID1A and the CC of SMARCC (Fig. 4A). The α9 helix of SMARCD1 packs against the α1 helix of SMARCE1 and a loop of ARID1A, generating a “pillar” that connects to the bridge (Figs. 2 and 4A). The α10, α11, and α12 helices of SMARCD1 interact with the SANTb domain and pre-HSA with the α12 helix protruding out of the intersection of the head, bridge, and thumb (Fig. 4, A and C). Thus, SMARCD1 and SMARCE1 assist SMARCC1 and SMARCC2 in organizing the base module.

Comparison of BAF-NCP and RSC-NCP complex structures

The yeast RSC complex is the homolog of the mammalian PBAF complex, consisting of ARID2 (instead of ARID1A and ARID1B) and a PBAF-specific subunit, polybromo-1 (PBRM1, or BAF180). Comparison of our BAF-NCP structure and a recently published yeast RSC-NPC structure (28) revealed a similar nucleosome-binding mode (fig. S10). Unexpectedly, the nucleosome is mainly bound by the ATPase domain, and the αC helix of Sfh1 is invisible in the RSC-NCP structure (28). In contrast, the nucleosome is sandwiched by BAF with H2A-H2B dimer stably associated with the αC helix of SMARCB1. The two complexes may represent different conformational states, or this difference might result from different experimental conditions. Structural comparison also revealed considerable differences in the base module organization. ARID1A, SMARCC1, SMARCC2, SMARCD1, and SMARCE1 exhibit conformations distinct from their counterparts in the RSC complex. The comparison also suggested that Rsc7 and Htl1 in RSC are equivalents of DPF2 and SMARCE1 of BAF, respectively. BAF has no histone tail–binding lobe, which may exist exclusively in RSC and PBAF complexes. Homologs of yeast DNA interaction subunits, RSC3 and RSC30, do not exist in mammals, suggesting that PBAF is also structurally different from RSC.

Model of chromatin remodeling of the BAF complex

Our structural and biochemical analyses illustrated the mechanisms of organization and nucleosome recognition of the human BAF complex, the prototype mSWI/SNF remodeler (Fig. 5A). BAF ejects the nucleosome and creates and maintains NDRs that are essential for transcription. Two non–mutually exclusive models of nucleosome ejection have been proposed (2). The BAF-NCP structure could fit both models (Fig. 5B). In the first model, the BAF-bound histone octamer, and more possibly the H2A-H2B dimer, could be evicted because of DNA tension resulting from either strong DNA translocation (Fig. 5B, model 1a; independent of the adjacent nucleosome) or collision with the adjacent nucleosome (Fig. 5B, model 1b). In the second model, BAF sandwiches the nucleosome to ensure that the ATPase domain stably engages with DNA and “peels” the DNA off of the adjacent nucleosome. In both cases, the two nucleosome-sandwiching regions, the SnAC domain of SMARCA4 and the αC helix of SMARCB1, likely play important roles in nucleosome ejection, which has been experimentally demonstrated (14, 16, 28, 33). The DNA detachment may facilitate efficient DNA translocation of the BAF-associated nucleosome because of fewer DNA–histone contacts. The chromatin ejection may also be promoted by the DNA-interacting ARID domain of ARID1A and/or acetylated histone tail–binding bromodomain of SMARCA4, which were not observed in the cryo-EM map, owing to flexibility.

Fig. 5 Models of chromatin remodeling of the BAF complex.

(A) BAF subunits are shown in cartoon model. The indicated paralogs of BAF subunits are mutually exclusive in the BAF complex. The nucleosome is sandwiched by SMARCA2/4 and SMARCB1. The ATPase of SMARCA2/4 grasps nucleosomal DNA and generates DNA translocation in an ATP-dependent manner. PHD, plant homeodomain; HMG, high-mobility group. (B) The BAF-NCP structure could fit two nonexclusive models of nucleosome ejection. The histone octamer or dimer could be ejected from BAF-bound nucleosome (model 1) or the adjacent nucleosome (model 2). The ejection could occur independent of (model 1a) or dependent upon (model 1b) the adjacent nucleosome.

Supplementary Materials

Materials and Methods

Supplementary Text

Figs. S1 to S10

Tables S1 and S2

References (4161)

Movies S1 to S6

Data S1 and S2

References and Notes

Acknowledgments: We thank the Center of Cryo-Electron Microscopy, Fudan University; the Center of Cryo-Electron Microscopy, ShanghaiTech University; the Center for Biological Imaging of Institute of Biophysics (IBP) of Chinese Academy of Sciences (CAS); and the National Center for Protein Science Shanghai (NCPSS) for supporting cryo-EM data collection and data analyses. We also thank the Biomedical Core Facility, Fudan University, for supporting mass spectrometry analyses. Funding: This work was supported by grants from the National Key R&D Program of China (2016YFA0500700), the National Natural Science Foundation of China (31830107, 31821002, and 31425008), the National Ten-Thousand Talent Program (Y.X.), the National Program for Support of Top-Notch Young Professionals (Y.X.), the Shanghai Municipal Science and Technology Major Project (2017SHZDZX01), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB08000000). Author contributions: S.H. prepared the samples for structural and biochemical analyses with help from J.Y., J.L., X.W., and B.L. Z.W. and Y.T. performed EM analyses and model building with help from Z.Y. Y.X. and S.H. wrote the manuscript. Y.X. supervised the project. Competing interests: The authors declare no competing interests. Data and materials availability: Cryo-EM maps have been deposited in the Electron Microscopy Data Bank (EMDB) under accession numbers EMD-0968 (base module), EMD-0969 (ARP module), EMD-0970 (ATPase-NCP with αC of SMARCB1), EMD-0972 (ATPase-NCP with detached DNA), EMD-0974 (BAF-NCP, 3.7 Å), EMD-0971 (BAF-NCP, 6.6 Å), and EMD-0973 (BAFADP-NCP). Atomic coordinates for the base module and BAF-NCP have been deposited in the Protein Data Bank under IDs 6LTH and 6LTJ, respectively.

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