SARS-CoV核蛋白和宿主细胞相互作用的初步研究

寻找与SARS-CoV核蛋白相互作用的宿主细胞蛋白,从而探索SARS-CoV的致病机理。可溶性表达SARS-CoV核蛋白,利用His标签和离子交换层析对表达的蛋白进行了纯化,获得较纯的可溶性核蛋白。再将SPR/BIA技术与MALDI-TOF MS技术结合起来,使用SPR生物传感芯片作为亲和吸附的表面,分别捕获2BS细胞和A549细胞裂解液中与SARS-CoV核蛋白相互作用的细胞蛋白,收集足够量的相互作用蛋白,再利用MALDI-TOF-MS分析获得蛋白的性质。结果鉴定出与SARS-CoV核蛋白相互作用的蛋白:26S蛋白酶调节亚单位S10B(蛋白酶体亚单位p42)(蛋白酶体26S亚单位ATPase 6)(P62333),属于泛素/蛋白酶体系统;目前国内外尚未见类似报道。此研究初步发现了一种与SARS-CoV核蛋白在细胞外相互作用的蛋白,但这种相互作用在SARS-CoV感染及SARS的发生发展中发挥的作用还有待于深入研究和探索。     

Mutants of human ACE2 differentially promote SARS-CoV and SARS-CoV-2 spike mediated infection

SARS-CoV and SARS-CoV-2 encode spike proteins that bind human ACE2 on the cell surface to enter target cells during infection. A small fraction of humans encode variants of ACE2, thus altering the biochemical properties at the protein interaction interface. These and other ACE2 coding mutants can reveal how the spike proteins of each virus may differentially engage the ACE2 protein surface during infection. We created an engineered HEK 293T cell line for facile stable transgenic modification, and expressed the major human ACE2 allele or 28 of its missense mutants, 24 of which are possible through single nucleotide changes from the human reference sequence. Infection with SARS-CoV or SARS-CoV-2 spike pseudotyped lentiviruses revealed that high ACE2 cell-surface expression could mask the effects of impaired binding during infection. Drastically reducing ACE2 cell surface expression revealed a range of infection efficiencies across the panel of mutants. Our infection results revealed a non-linear relationship between soluble SARS-CoV-2 RBD binding to ACE2 and pseudovirus infection, supporting a major role for binding avidity during entry. While ACE2 mutants D355N, R357A, and R357T abrogated entry by both SARS-CoV and SARS-CoV-2 spike proteins, the Y41A mutant inhibited SARS-CoV entry much more than SARS-CoV-2, suggesting differential utilization of the ACE2 side-chains within the largely overlapping interaction surfaces utilized by the two CoV spike proteins. These effects correlated well with cytopathic effects observed during SARS-CoV-2 replication in ACE2-mutant cells. The panel of ACE2 mutants also revealed altered ACE2 surface dependencies by the N501Y spike variant, including a near-complete utilization of the K353D ACE2 variant, despite decreased infection mediated by the parental SARS-CoV-2 spike. Our results clarify the relationship between ACE2 abundance, binding, and infection, for various SARS-like coronavirus spike proteins and their mutants, and inform our understanding for how changes to ACE2 sequence may correspond with different susceptibilities to infection.     

In vitro biochemical and thermodynamic characterization of nucleocapsid protein of SARS

The major biochemical and thermodynamic features of nucelocapsid protein of SARS coronavirus (SARS_NP) were characterized by use of non-denatured gel electrophoresis, size-exclusion chromatographic and surface plasmon resonance (SPR) techniques. The results showed that SARS_NP existed in vitro as oligomer, more probably dimer, as the basic functional unit. This protein shows its maximum conformational stability near pH 9.0, and it seems that its oligomer dissociation and protein unfolding occur simultaneously. Thermal-induced unfolding for SARS_NP was totally irreversible. Both the thermal and chemical denaturant-induced denaturation analyses showed that oligomeric SARS_NP unfolds and refolds through a two-state model, and the electrostatic interactions among the charge groups of SARS_NP made a significant contribution to its conformational stability.     

Inhibition of SARS pseudovirus cell entry by lactoferrin binding to heparan sulfate proteoglycans

It has been reported that lactoferrin (LF) participates in the host immune response against Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) invasion by enhancing NK cell activity and stimulating neutrophil aggregation and adhesion. We further investigated the role of LF in the entry of SARS pseudovirus into HEK293E/ACE2-Myc cells. Our results reveal that LF inhibits SARS pseudovirus infection in a dose-dependent manner. Further analysis suggested that LF was able to block the binding of spike protein to host cells at 4°C, indicating that LF exerted its inhibitory function at the viral attachment stage. However, LF did not disrupt the interaction of spike protein with angiotensin-converting enzyme 2 (ACE2), the functional receptor of SARS-CoV. Previous studies have shown that LF colocalizes with the widely distributed cell-surface heparan sulfate proteoglycans (HSPGs). Our experiments have also confirmed this conclusion. Treatment of the cells with heparinase or exogenous heparin prevented binding of spike protein to host cells and inhibited SARS pseudovirus infection, demonstrating that HSPGs provide the binding sites for SARS-CoV invasion at the early attachment phase. Taken together, our results suggest that, in addition to ACE2, HSPGs are essential cell-surface molecules involved in SARS-CoV cell entry. LF may play a protective role in host defense against SARS-CoV infection through binding to HSPGs and blocking the preliminary interaction between SARS-CoV and host cells. Our findings may provide further understanding of SARS-CoV pathogenesis and aid in treatment of this deadly disease.     

Interactions between M protein and other structural proteins of severe,acute respiratory syndrome-associated coronavirus

Severe acute respiratory syndrome-associated coronavirus (SARS-CoV) structural proteins (S, E, M, and NC) localize in different subcellular positions when expressed individually. However, SARS-CoV M protein is co-localized almost entirely with S, E, or NC protein when co-expressed in the cells. On the other hand, only partial co-localization was observed when S and E, S and NC, or E and NC were co-expressed in the cells. Interactions between SARS-CoV M and other structural proteins but not interactions between S and E, S and NC, or E and NC were further demonstrated by co-immunoprecipitation assay. These results indicate that SARS-CoV M protein, similar to the M proteins of other coronaviruses, plays a pivotal role in virus assembly. The cytoplasmic C-terminus domain of SARS-CoV M protein was responsible for binding to NC protein. Multiple regions of M protein interacted with E and S proteins. A model for the interactions between SARS-CoV M protein and other structural proteins is proposed. This study helps us better understand protein-protein interactions during viral assembly of SARS-CoV. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

SR-rich motif plays a pivotal role in recombinant SARS coronavirus nucleocapsid protein multimerization

The nucleocapsid (N) protein of SARS coronavirus (SARS-CoV) is reported to function in encapsidating the viral genomic RNA into helical nucleocapsid, and its self-association is believed to be vital in coating the viral genomic RNA. Characterization of SARS-CoV N multimerization may thereby help us better understand the coronavirus assembly. In the current work, using the yeast two-hybrid technique, an unexpected interaction between residues 1-210 and 211-290 (central region) of the SARS-CoV N protein was detected, and SPR results further revealed that the SR-rich motif (amino acids 183-197) of SARS-CoV N protein is responsible for such an interaction. Chemical cross-linking and gel-filtration analyses indicated that the residues 283-422 of the SARS-CoV N protein have multimeric ability, although the full-length N protein is prone to exist predominantly as dimers. In addition, the multimeric ability of the C-terminal domain of SARS-CoV N protein could be weakened by the SR-rich motif interaction with the central region (amino acids 211-290). All of these data suggested that the SR-rich motif of the SARS-CoV N protein might play an import role in the transformation of the SARS-CoV N protein between the dimer and multimer during its binding to its central region for self-association or dissociation. This current paper will hopefully provide some new ideas in studying SARS-CoV N multimerization.     

Molecular and biological characterization of human monoclonal antibodies binding to the spike and nucleocapsid proteins of severe acute respiratory syndrome coronavirus

Human monoclonal antibodies (MAbs) were selected from semisynthetic antibody phage display libraries by using whole irradiated severe acute respiratory syndrome (SARS) coronavirus (CoV) virions as target. We identified eight human MAbs binding to virus and infected cells, six of which could be mapped to two SARS-CoV structural proteins: the nucleocapsid (N) and spike (S) proteins. Two MAbs reacted with N protein. One of the N protein MAbs recognized a linear epitope conserved between all published human and animal SARS-CoV isolates, and the other bound to a nonlinear N epitope. These two N MAbs did not compete for binding to SARS-CoV. Four MAbs reacted with the S glycoprotein, and three of these MAbs neutralized SARS-CoV in vitro. All three neutralizing anti-S MAbs bound a recombinant S1 fragment comprising residues 318 to 510, a region previously identified as the SARS-CoV S receptor binding domain; the nonneutralizing MAb did not. Two strongly neutralizing anti-S1 MAbs blocked the binding of a recombinant S fragment (residues 1 to 565) to SARS-CoV-susceptible Vero cells completely, whereas a poorly neutralizing S1 MAb blocked binding only partially. The MAb ability to block S1-receptor binding and the level of neutralization of the two strongly neutralizing S1 MAbs correlated with the binding affinity to the S1 domain. Finally, epitope mapping, using recombinant S fragments (residues 318 to 510) containing naturally occurring mutations, revealed the importance of residue N479 for the binding of the most potent neutralizing MAb, CR3014. The complete set of SARS-CoV MAbs described here may be useful for diagnosis, chemoprophylaxis, and therapy of SARS-CoV infection and disease.     

Severe acute respiratory syndrome coronavirus infection of human ciliated airway epithelia: role of ciliated cells in viral spread in the conducting airways of the lungs

Severe acute respiratory syndrome coronavirus (SARS-CoV) emerged in 2002 as an important cause of severe lower respiratory tract infection in humans, and in vitro models of the lung are needed to elucidate cellular targets and the consequences of viral infection. The SARS-CoV receptor, human angiotensin 1-converting enzyme 2 (hACE2), was detected in ciliated airway epithelial cells of human airway tissues derived from nasal or tracheobronchial regions, suggesting that SARS-CoV may infect the proximal airways. To assess infectivity in an in vitro model of human ciliated airway epithelia (HAE) derived from nasal and tracheobronchial airway regions, we generated recombinant SARS-CoV by deletion of open reading frame 7a/7b (ORF7a/7b) and insertion of the green fluorescent protein (GFP), resulting in SARS-CoV GFP. SARS-CoV GFP replicated to titers similar to those of wild-type viruses in cell lines. SARS-CoV specifically infected HAE via the apical surface and replicated to titers of 10(7) PFU/ml by 48 h postinfection. Polyclonal antisera directed against hACE2 blocked virus infection and replication, suggesting that hACE2 is the primary receptor for SARS-CoV infection of HAE. SARS-CoV structural proteins and virions localized to ciliated epithelial cells. Infection was highly cytolytic, as infected ciliated cells were necrotic and shed over time onto the luminal surface of the epithelium. SARS-CoV GFP also replicated to a lesser extent in ciliated cell cultures derived from hamster or rhesus monkey airways. Efficient SARS-CoV infection of ciliated cells in HAE provides a useful in vitro model of human lung origin to study characteristics of SARS-CoV replication and pathogenesis.     

Pathways of cross-species transmission of synthetically reconstructed zoonotic severe acute respiratory syndrome coronavirus

Zoonotic severe acute respiratory syndrome coronavirus (SARS-CoV) likely evolved to infect humans by a series of transmission events between humans and animals in markets in China. Virus sequence data suggest that the palm civet served as an amplification host in which civet and human interaction fostered the evolution of the epidemic SARS Urbani strain. The prototypic civet strain of SARS-CoV, SZ16, was isolated from a palm civet but has not been successfully cultured in vitro. To propagate a chimeric recombinant SARS-CoV bearing an SZ16 spike (S) glycoprotein (icSZ16-S), we constructed cell lines expressing the civet ortholog (DBT-cACE2) of the SARS-CoV receptor (hACE2). Zoonotic SARS-CoV was completely dependent on ACE2 for entry. Urbani grew with similar kinetics in both the DBT-cACE2 and the DBT-hACE2 cells, while icSZ16-S only grew in DBT-cACE2 cells. The SZ16-S mutant viruses adapted to human airway epithelial cells and displayed enhanced affinity for hACE2 but exhibited severe growth defects in the DBT-cACE2 cells, suggesting that the evolutionary pathway that promoted efficient hACE2 interactions simultaneously abolished efficient cACE2 interactions. Structural modeling predicted two distinct biochemical interaction networks by which zoonotic receptor binding domain architecture can productively engage hACE2, but only the Urbani mutational repertoire promoted efficient usage of both hACE2 and cACE2 binding interfaces. Since dual species tropism was preserved in Urbani, it is likely that the virus evolved a high affinity for cACE2/hACE2 receptors through adaptation via repeated passages between human and civet hosts. Furthermore, zoonotic SARS-CoV was variably neutralized by antibodies that were effective against the epidemic strain, highlighting their utility for evaluating passive immunization efficacy.     

Discovering severe acute respiratory syndrome coronavirus 3CL protease inhibitors: virtual screening, surface plasmon resonance, and fluorescence resonance energy transfer assays

An integrated system has been developed for discovering potent inhibitors of severe acute respiratory syndrome coronavirus 3C-like protease (SARS-CoV 3CL(pro)) by virtual screening correlating with surface plasmon resonance (SPR) and fluorescence resonance energy transfer (FRET) technologies-based assays. The authors screened 81,287 small molecular compounds against SPECS database by virtual screening; 256 compounds were subsequently selected for biological evaluation. Through SPR technology-based assay, 52 from these 256 compounds were discovered to show binding to SARS-CoV 3CL(pro). The enzymatic inhibition activities of these 52 SARS-CoV 3CL(pro) binders were further applied to FRET-based assay, and IC(50) values were determined. Based on this integrated assay platform, 8 new SARS-CoV 3CL(pro) inhibitors were discovered. The fact that the obtained IC(50) values for the inhibitors are in good accordance with the discovered dissociation equilibrium constants (K(D)s) assayed by SPR implied the reliability of this platform. Our current work is hoped to supply a powerful approach in the discovery of potent SARS-CoV 3CL(pro) inhibitors, and the determined inhibitors could be used as possible lead compounds for further research.     

T-cell epitopes in severe acute respiratory syndrome (SARS) coronavirus spike protein elicit a specific T-cell immune response in patients who recover from SARS

The immunogenicity of HLA-A2-restricted T-cell epitopes in the S protein of the Severe acute respiratory syndrome coronavirus (SARS-CoV) and of human coronavirus strain 229e (HCoV-229e) was analyzed for the elicitation of a T-cell immune response in donors who had fully recovered from SARS-CoV infection. We employed online database analysis to compare the differences in the amino acid sequences of the homologous T epitopes of HCoV-229e and SARS-CoV. The identified T-cell epitope peptides were synthesized, and their binding affinities for HLA-A2 were validated and compared in the T2 cell system. The immunogenicity of all these peptides was assessed by using T cells obtained from donors who had fully recovered from SARS-CoV infection and from healthy donors with no history of SARS-CoV infection. HLA-A2 typing by indirect immunofluorescent antibody staining showed that 51.6% of SARS-CoV-infected patients were HLA-A2 positive. Online database analysis and the T2 cell binding test disclosed that the number of HLA-A2-restricted immunogenic epitopes of the S protein of SARS-CoV was decreased or even lost in comparison with the homologous sequences of the S protein of HCoV-229e. Among the peptides used in the study, the affinity of peptides from HCoV-229e (H77 and H881) and peptides from SARS-CoV (S978 and S1203) for binding to HLA-A2 was higher than that of other sequences. The gamma interferon (IFN-gamma) release Elispot assay revealed that only SARS-CoV-specific peptides S1203 and S978 induced a high frequency of IFN-gamma-secreting T-cell response in HLA-A2(+) donors who had fully recovered from SARS-CoV infection; such a T-cell epitope-specific response was not observed in HLA-A2(+) healthy donors or in HLA-A2(-) donors who had been infected with SARS-CoV after full recovery. Thus, T-cell epitopes S1203 and S978 are immunogenic and elicit an overt specific T-cell response in HLA-A2(+) SARS-CoV-infected patients.     

Molecular dynamics simulations of various coronavirus main proteinases

In this study, two homology models (denoted as MproST and MproSH) of main proteinase (Mpro) from the novel coronavirus associated with severe acute respiratory syndrome (SARS-CoV) were constructed based on the crystal structures of Mpro from transmissible gastroenteritis coronavirus (TGEV) (MproT) and human coronavirus HcoV-229E (MproH), respectively. Both MproST and MproSH exhibit similar folds as their respective template proteins. These homology models reveal three distinct functional domains as well as an intervening loop connecting domains II and III as found in both template proteins. A catalytic cleft containing the substrate binding sites S1 and S2 between domains I and II are also observed. S2 undergoes more significant structural fluctuation than S1 during the 400 ps molecular dynamics simulations because it is located at the open mouth of the catalytic cleft, while S1 is situated in the very bottom of this cleft. The thermal unfolding of these proteins begins at domain III, where the structure is least conserved among these proteins. Mpro may still maintain its proteolytic activity while it is partially unfolded. The electrostatic interaction between Arg40 and Asp186 plays an important role in maintaining the structural integrity of both S1 and S2.     

The unique C termini of orthopoxvirus gamma interferon binding proteins are essential for ligand binding

The orthopoxviruses ectromelia virus (ECTV) and vaccinia virus (VACV) express secreted gamma interferon binding proteins (IFN-gammaBPs) with homology to the ligand binding domains of the host's IFN-gamma receptor (IFN-gammaR1). Homology between these proteins is limited to the extracellular portions of the IFN-gammaR1 and the first approximately 200 amino acids of the IFN-gammaBPs. The remaining 60 amino acids at the C termini of the IFN-gammaBPs contain a single cysteine residue shown to be important in covalent dimerization of the secreted proteins. The function of the remaining C-terminal domain (CTD) has remained elusive, yet this region is conserved within all orthopoxvirus IFN-gammaBPs. Using a series of C-terminal deletion constructs, we have determined that the CTD is essential for IFN-gamma binding despite having no predicted homology to the IFN-gammaR1. Truncation of the ECTV IFN-gammaBP by more than two amino acid residues results in a complete loss of binding activity for both murine IFN-gamma and human IFN-gamma (hIFN-gamma), as measured by surface plasmon resonance (SPR) and bioassay. Equivalent truncation of the VACV IFN-gammaBP resulted in comparable loss of hIFN-gamma binding activity by SPR. Full-length IFN-gammaBPs were observed to form higher-ordered structures larger than the previously reported dimers. Mutants that were unable to bind IFN-gamma with high affinity in SPR experiments failed to assemble into these higher-ordered structures and migrated as dimers. We conclude that the unique CTD of orthopoxvirus IFN-gammaBPs is important for the assembly of covalent homodimers as well as the assembly of higher-ordered structures essential for IFN-gamma binding.     

Identifying epitopes responsible for neutralizing antibody and DC-SIGN binding on the spike glycoprotein of the severe acute respiratory syndrome coronavirus

The severe acute respiratory syndrome-associated coronavirus (SARS-CoV) uses dendritic cell-specific ICAM-3 grabbing nonintegrin (DC-SIGN) to facilitate cell entry via cellular receptor-angiotensin-converting enzyme 2. For this project, we used recombinant baculoviruses expressing different lengths of SARS-CoV spike (S) protein in a capture assay to deduce the minimal DC-SIGN binding region. Our results identified the region location between amino acid (aa) residues 324 to 386 of the S protein. We then generated nine monoclonal antibodies (MAbs) against the S protein to map the DC-SIGN-binding domain using capture assays with pseudotyped viruses and observed that MAb SIa5 significantly blocked S protein-DC-SIGN interaction. An enhancement assay using the HKU39849 SARS-CoV strain and human immature dendritic cells confirmed our observation. Data from a pepscan analysis and M13 phage peptide display library system mapped the reactive MAb SIa5 epitope to aa residues 363 to 368 of the S protein. Results from a capture assay testing three pseudotyped viruses with mutated N-linked glycosylation sites of the S protein indicate that only two pseudotyped viruses (N330Q and N357Q, both of which lost glycosylation sites near the SIa5 epitope) had diminished DC-SIGN-binding capacity. We also noted that MAb SIb4 exerted a neutralizing effect against HKU39849; its reactive epitope was mapped to aa residues 435 to 439 of the S protein. We offer the data to facilitate the development of therapeutic agents and preventive vaccines against SARS-CoV infection.     

Specific binding of alpha-macroglobulin to complement-type repeat CR4 of the low-density lipoprotein receptor-related protein

The low-density lipoprotein receptor-related protein (LRP) is a large surface receptor that mediates binding and internalization of a large number of structurally and functionally unrelated ligands. The ligand binding sites are located in clusters of complement-type repeats (CR), where the general absence of mutual binding competition suggests that different ligands map to distinct sites. Binding of alpha(2)-macroglobulin-protease complexes to the LRP is mediated by the receptor binding domain (RBD) of alpha(2)-macroglobulin (alpha(2)M). To determine the major binding epitope(s) in the LRP, we generated a complete set of tandem CR proteins spanning the second cluster of CR domains, and identified a binding site for alpha(2)M in the N-terminal part of the cluster comprising CR3-CR6, using ligand blotting and surface plasmon resonance (SPR) analysis. The specific site involved in alpha(2)M recognition resides in the fourth CR domain, CR4, whereas another site is identified in CR5. An acidic epitope in CR4 is identified as important for binding alpha(2)M by mutagenesis and SPR analysis. The formation of the complex between the rat alpha(1)-macroglobulin RBD and CR domain pairs is characterized by analytical size-exclusion chromatography, which demonstrates a sufficiently strong interaction between the alpha(1)M RBD and CR34 or CR45 for the isolation of a complex.     

Specific asparagine-linked glycosylation sites are critical for DC-SIGN- and L-SIGN-mediated severe acute respiratory syndrome coronavirus entry

Severe acute respiratory syndrome (SARS) is caused by a newly emerged coronavirus (CoV) designated SARS-CoV. The virus utilizes angiotensin-converting enzyme 2 (ACE2) as the primary receptor. Although the idea is less clear and somewhat controversial, SARS-CoV is thought to use C-type lectins DC-SIGN and/or L-SIGN (collectively referred to as DC/L-SIGN) as alternative receptors or as enhancer factors that facilitate ACE2-mediated virus infection. In this study, the function of DC/L-SIGN in SARS-CoV infection was examined in detail. The results of our study clearly demonstrate that both proteins serve as receptors independently of ACE2 and that there is a minimal level of synergy between DC/L-SIGN and ACE2. As expected, glycans on spike (S) glycoprotein are important for DC/L-SIGN-mediated virus infection. Site-directed mutagenesis analyses have identified seven glycosylation sites on the S protein critical for DC/L-SIGN-mediated virus entry. They include asparagine residues at amino acid positions 109, 118, 119, 158, 227, 589, and 699, which are distinct from residues of the ACE2-binding domain (amino acids 318 to 510). Amino acid sequence analyses of S proteins encoded by viruses isolated from animals and humans suggest that glycosylation sites N227 and N699 have facilitated zoonotic transmission.     

Characterization of the heptad repeat regions, HR1 and HR2, and design of a fusion core structure model of the spike protein from severe acute respiratory syndrome (SARS) coronavirus

Severe acute respiratory syndrome coronavirus (SARS-CoV) is a newly emergent virus responsible for a worldwide epidemic in 2003. The coronavirus spike proteins belong to class I fusion proteins, and are characterized by the existence of two heptad repeat (HR) regions, HR1 and HR2. The HR1 region in coronaviruses is predicted to be considerably longer than that in other type I virus fusion proteins. Therefore the exact binding sequence to HR2 from the HR1 is not clear. In this study, we defined the region of HR1 that binds to HR2 by a series of biochemical and biophysical measures. Subsequently the defined HR1 (902-952) and HR2 (1145-1184) chains, which are different from previously defined binding regions, were linked together by a flexible linker to form a single-chain construct, 2-Helix. This protein was expressed in Escherichia coli and forms a typical six-helix coiled coil bundle. Highly conserved HR regions between mouse hepatitis virus (MHV) and SARS-CoV spike proteins suggest a similar three-dimensional structure for the two fusion cores. Here, we constructed a homology model for SARS coronavirus fusion core based on our biochemical analysis and determined the MHV fusion core structure. We also propose an important target site for fusion inhibitor design and several strategies, which have been successfully used in fusion inhibitor design for human immunodeficiency virus (HIV), for the treatment of SARS infection.     

Crystal structure of the human U4/U6 small nuclear ribonucleoprotein particle-specific SnuCyp-20, a nuclear cyclophilin

The cyclophilin SnuCyp-20 is a specific component of the human U4/U6 small nuclear ribonucleoprotein particle involved in the nuclear splicing of pre-mRNA. It stably associates with the U4/U6-60kD and -90kD proteins, the human orthologues of the Saccharomyces cerevisiae Prp4 and Prp3 splicing factors. We have determined the crystal structure of SnuCyp-20 at 2.0-A resolution by molecular replacement. The structure of SnuCyp-20 closely resembles that of human cyclophilin A (hCypA). In particular, the catalytic centers of SnuCyp-20 and hCypA superimpose perfectly, which is reflected by the observed peptidyl-prolyl-cis/trans-isomerase activity of SnuCyp-20. The surface properties of both proteins, however, differ significantly. Apart from seven additional amino-terminal residues, the insertion of five amino acids in the loop alpha1-beta3 and of one amino acid in the loop alpha2-beta8 changes the conformations of both loops. The enlarged loop alpha1-beta3 is involved in the formation of a wide cleft with predominantly hydrophobic character. We propose that this enlarged loop is required for the interaction with the U4/U6-60kD protein.