The two envelope glycoproteins and the viral nucleocapsid of the coronavirus A59 were isolated by solubilization of the viral membrane with Nonidet P-40 at 4 degrees C followed by sucrose density gradient sedimentation. Isolated E2 consisted of rosettes of peplomers, whereas E1, the membrane glycoprotein, was irregular and amorphous. Under certain conditions significant interactions occurred between components of Nonidet P-40-disrupted virions. Incubation of the Nonidet P-40-disrupted virus at 37 degrees C resulted in formation of a complex between one of the viral glycoproteins, E1, and the viral nucleocapsid. This was caused by a temperature-dependent conformational change in E1, resulting in aggregation of E1 and interaction with the viral RNA in the nucleocapsid. E1 also bound rRNA. The E1-nucleocapsid complexes can be distinguished on sucrose and Renografin density gradients from native viral nucleocapsids. The separation of the membrane glycoprotein E1 from the peplomeric glycoprotein E2 permitted preparation of antisera against these isolated proteins. A model is proposed for the arrangement of the three major structural proteins in the coronavirus A59 virion in relation to the viral envelope and RNA. Images

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Isolation of coronavirus envelope glycoproteins and interaction with the viral nucleocapsid.

Publisher Summary Coronaviruses have recently emerged as an important group of animal and human pathogens that share a distinctive replicative cycle. Some of the unique characteristics in the replication of coronaviruses include generation of a 3' coterminal-nested set of five or six subgenomic mRNAs, each of which appears to direct the synthesis of one protein. Two virus-specific RNA polymerase activities have been identified. Many of the distinctive features of coronavirus infection and coronavirus-induced diseases may result from the properties of the two coronavirus glycoproteins. The intracellular budding site, which may be important in the establishment and maintenance of persistent infections, appears to be due to the restricted intracytoplasmic migration of the E1 glycoprotein, which acts as a matrix-like transmembrane glycoprotein. E1 also exhibits distinctive behavior by self-aggregating on heating at 100°C in sodium dodecyl sulfate (SDS) and by its interaction with RNA in the viral nucleocapsid. The E1 of mouse hepatitis virus (MHV) is an O-linked glycoprotein, unlike most other viral glycoproteins. Thus, the coronavirus system may be a useful model for the study of synthesis, glycosylation, and transport of O-linked cellular glycoproteins.

The molecular biology of coronaviruses

Coronaviruses were recognized as a group in 1968 primarily on the basis of their characte ristic morphology as seen in the electron microscope (Tyrrell et al. 1968). Since that time our knowledge of the structure and replication of these viruses has increased steadily and has been periodically reviewed (McIntosh 1974; Tyrrell et al. 1978; Robb and Bond 1979a). The basis for this review, which concentrates on the molecular biology of coronaviruses, is principally the new data which has become available in the last 2 years. The pathogeni-city of these viruses, which are associated with many diseases of clinical importance in animals and humans, is the subject of the accompanying article.

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The Structure and Replication of Coronaviruses

I. Structural proteins: effects of preparative conditions on the migration of protein in polyacrylamide gels.

Summary
Avian infectious bronchitis virus (IBV) was grown and radiolabelled with 35S-methionine, 3H-leucine and 3H-glucosamine in de-embryonated chicken eggs. Approximately 12 different polypeptides were clearly detected by SDS-polyacrylamide gel electrophoresis of virus preparations. Growth of IBV in chorioallantoic membrane cells labelled with 35S-methionine indicated that most of these polypeptides, and additional ones, some of which were glycosylated, were host components. Five polypeptides appeared to be virus-coded, with apparent mol. wt. of 94 × 103, 84 × 103, 54 × 103, 30 × 103 and 28 × 103. Four of these, p94, p84, p30 and p28, were glycosylated. The virion spikes appeared to be composed of p94 and p84, while p30 and p28 were partially embedded in the virion membrane. By analogy with other reports, p54 is the nucleocapsid polypeptide.

Structural polypeptides of coronavirus IBV.

Egg-grown infectious bronchitis virus, strain Beaudette, was concentrated and centrifuged on sucrose density gradients to separate the virus into five peaks with densities of 1.144, 1.160, 1.172, 1.191 and 1.218 g/cm3. All peaks retained infectivity, complement fixation activity and were labelled with 3H-uridine. Morphologically the densest peak consisted of very large virus particles and amorphous material, the other peaks consisted of mainly intact particles although small differences in size and pleomorphism were seen. Polyacrylamide gel electrophoresis of material from the density gradient peaks revealed four major polypeptides and at least 10 minor polypeptides. The proportions of the polypeptides were approximately similar for all peaks with the exception of the densest peak in which the major polypeptides were greatly reduced. The four major polypeptides had approximate molecular weights of 1. 52,000, 2. 45,000, 3. 34,000, 4. 32,000. The major polypeptides 1 and 4 were shown to be glycosylated as were two of the minor polypeptides.

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Heterogeneity of infectious bronchitis virus grown in eggs.

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Heterogeneity of infectious bronchitis virus grown in eggs.