Posttranscriptional regulation of a class of human cytomegalovirus phosphoproteins encoded by an early transcription unit

Abstract

We have further characterized and determined the origins of a family of nuclear phosphoproteins of 84, 50, 43, and 34 kilodaltons (kDa) encoded by a class of early transcripts arising from the adjacent EcoRI fragments R and d (map units 0.682 to 0.713) of the strain AD169 human cytomegalovirus genome. These RNAs have a complex spliced structure with common 5' and internal exons and alternative 3' exons with coterminal 3' ends. At early times, two fully processed species of 2.1 and 2.2 kilobases (kb) predominated. As the infection progressed to late times, there was a decrease in splicing of the RNA, generating larger transcripts 2.5 to 2.65 kb in size, which corresponded to the species which had spliced out only the first intron, as well as the completely unspliced transcript. We previously reported that the 34-kDa protein could be derived from a transcript which had failed to splice out the first intron (D. A. Wright, S. I. Staprans, and D. H. Spector, J. Virol. 62:331-340, 1988), but the origin of the other proteins was unclear. cDNA cloning has shown that the 2.1-, 2.2-, and 2.5-kb RNAs encode the 50-, 43-, and 84-kDa proteins, respectively. The shift in the splicing pattern of these RNAs with time revealed a posttranscriptional control mechanism which results in the differential accumulation of individual proteins within this family of nuclear phosphoproteins. Expression of the 84-, 43-, and 34-kDa proteins correlated well with the steady-state concentrations of their respective mRNAs. The 50-kDa protein, however, was not expressed in abundance until late times, despite the presence of the 2.1-kb mRNA in the cytoplasm at early times, suggesting a secondary level of posttranscriptional regulation for this protein. Full expression of the RNAs and proteins was dependent on continuing viral DNA synthesis. Accumulation of the 50-kDa protein was found to be particularly sensitive to the state of viral DNA replication and could not be detected after inhibition of replication. Further analysis of these proteins revealed that each one had a unique pattern of serine phosphorylation. Although there was one common site of phosphorylation, most likely located within the amino-terminal shared region, even this site showed quantitative differences in the level of phosphorylation for each of the proteins. Analysis of the Towne strain and two recent independent clinical isolates of human cytomegalovirus has shown that this family of proteins is highly conserved among human cytomegaloviruses.