Cetacean morbillivirus (CeMV) is an emerging and highly infectious paramyxovirus that causes outbreaks in cetaceans and occasionally in pinnipeds, representing a major threat to biodiversity and conservation of endangered marine mammal populations in both hemispheres. As for all non-segmented, negative-sense, single-stranded RNA (ssRNA) viruses, the morbilliviral genome is enwrapped by thousands of nucleoprotein (N) protomers. Each bound to six ribonucleotides, N protomers assemble to form a helical ribonucleoprotein (RNP) complex that serves as scaffold for nucleocapsid formation and as template for viral replication and transcription. While the molecular details on RNP complexes elucidated in human measles virus (MeV) served as paradigm model for these processes in all members of the Morbillivirus genus, no structural information has been obtained from other morbilliviruses, nor has any CeMV structure been solved so far. We report the structure of the CeMV RNP complex, reconstituted in vitro upon binding of recombinant CeMV N to poly-adenine ssRNA hexamers and solved to 4.0 Å resolution by cryo-electron microscopy. In spite of the amino acid sequence similarity and consequently similar folding of the N protomer, the CeMV RNP complex exhibits different helical parameters as compared to previously reported MeV orthologs. The CeMV structure reveals exclusive interactions leading to more extensive protomer-RNA and protomer-protomer interfaces. We identified twelve residues, among those varying between CeMV strains, as putatively important for the stabilization of the RNP complex, which highlights the need to study the potential of CeMV N mutations that modulate nucleocapsid assembly to also affect viral phenotype and host adaptation.

鲸类麻疹病毒 (CeMV) 是一种新兴且具有高度传染性的副粘病毒，可导致鲸类动物和鳍足类动物爆发，对生物多样性和两个半球濒临灭绝的海洋哺乳动物种群的保护构成重大威胁。对于所有非分段、负义、单链 RNA (ssRNA) 病毒，麻疹病毒基因组被数以千计的核蛋白 (N) 原体包裹。每个与六个核糖核苷酸结合，N 原体组装形成螺旋核糖核蛋白 (RNP) 复合物，用作核衣壳形成的支架和病毒复制和转录的模板。虽然在人类麻疹病毒 (MeV) 中阐明的 RNP 复合物的分子细节作为麻疹病毒所有成员中这些过程的范例模型属，尚未从其他麻疹病毒中获得结构信息，迄今为止也没有解决任何CeMV结构。我们报告了 CeMV RNP 复合物的结构，在重组 CeMV N 与聚腺嘌呤 ssRNA 六聚体结合后在体外重组，并通过冷冻电子显微镜解决了 4.0 Å 的分辨率。尽管氨基酸序列相似并因此 N 原体的相似折叠，但与先前报道的 MeV 直向同源物相比，CeMV RNP 复合物表现出不同的螺旋参数。CeMV 结构揭示了导致更广泛的原体-RNA 和原体-原体界面的独特相互作用。我们确定了 12 个残基，在 CeMV 菌株之间变化的那些中，被认为对 RNP 复合物的稳定很重要，