Abstract
Particles of many paramyxoviruses include small amounts of proteins with a molecular weight of about 20 kDa. These proteins, termed “C”, are basic, have low amino acid homology and some secondary structure conservation. C proteins are encoded in alternative reading frames of the phosphoprotein gene. Some viruses express nested sets of C proteins that exert their functions in different locations: In the nucleus, they interfere with cellular transcription factors that elicit innate immune responses; in the cytoplasm, they associate with viral ribonucleocapsids and control polymerase processivity and orderly replication, thereby minimizing the activation of innate immunity. In addition, certain C proteins can directly bind to, and interfere with the function of, several cytoplasmic proteins required for interferon induction, interferon signaling and inflammation. Some C proteins are also required for efficient virus particle assembly and budding. C-deficient viruses can be grown in certain transformed cell lines but are not pathogenic in natural hosts. C proteins affect the same host functions as other phosphoprotein gene-encoded proteins named V but use different strategies for this purpose. Multiple independent systems to counteract host defenses may ensure efficient immune evasion and facilitate virus adaptation to new hosts and tissue environments.
Highlights
About 50 years ago, when the proteins from purified particles of different paramyxoviruses were first analyzed, several polypeptides were identified and their biological roles tentatively attributed [1,2,3,4,5]
When the synthesis of virus-specific proteins was analyzed in infected cells, two proteins not found in particles, and deemed nonstructural, were identified [7]
The transcriptional upregulation of IL-1β and IL-18 during RNA virus infections occurs after viral RNA sensing by retinoic acid inducible gene-I (RIG-I) and melanoma differentiation associated gene 5 (MDA-5); these cytokines are expressed as inactive pro-forms, which require further activation (Figure 4)
Summary
About 50 years ago, when the proteins from purified particles of different paramyxoviruses were first analyzed, several polypeptides were identified and their biological roles tentatively attributed [1,2,3,4,5]. When the synthesis of virus-specific proteins was analyzed in infected cells, two proteins not found in particles, and deemed nonstructural, were identified [7]. These proteins had MWs of about 36,000 and 20,000, respectively, and were labeled with the letters B and C. C proteins, or candidate C protein reading frames, are common but not ubiquitous in paramyxoviruses; they have been characterized in 26 of the 63 genomic reference sequences currently available in GenBank (Figure 1A; paramyxovirus genera expressing C proteins are shown in red and Table 1).
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