Virus-specific interaction between the human cytomegalovirus major capsid protein and the C terminus of the assembly protein precursor

We previously identified a minimal 12-amino-acid domain in the C terminus of the herpes simplex virus type 1 (HSV-1) scaffolding protein which is required for interaction with the HSV-1 major capsid protein. An alpha-helical structure which maximizes the hydropathicity of the minimal domain is required for the interaction. To address whether cytomegalovirus (CMV) utilizes the same strategy for capsid assembly, several glutathione S-transferase fusion proteins to the C terminus of the CMV assembly protein precursor were produced and purified from bacterial cells. The study showed that the glutathione S-transferase fusion containing 16 amino acids near the C-terminal end was sufficient to interact with the major capsid protein. Interestingly, no cross-interaction between HSV-1 and CMV could be detected. Mutation analysis revealed that a three-amino-acid region at the N-terminal side of the central Phe residue of the CMV interaction domain played a role in determining the viral specificity of the interaction. When this region was converted so as to correspond to that of HSV-1, the CMV assembly protein domain lost its ability to interact with the CMV major capsid protein but gained full interaction with the HSV-1 major capsid protein. To address whether the minimal interaction domain of the CMV assembly protein forms an alpha-helical structure similar to that in HSV-1, peptide competition experiments were carried out. The results showed that a cyclic peptide derived from the interaction domain with a constrained (alpha-helical structure competed for interaction with the major capsid protein much more efficiently than the unconstrained linear peptide. In contrast, a cyclic peptide containing an Ala substitution for the critical Phe residue did not compete for the interaction at all. The results of this study suggest that (i) CMV may have developed a strategy similar to that of HSV-1 for capsid assembly; (ii) the minimal interaction motif in the CMV assembly protein requires an alpha-helix for efficient interaction with the major capsid protein; and (iii) the Phe residue in the CMV minimal interaction domain is critical for interaction with the major capsid protein.     

The major immediate-early proteins IE1 and IE2 of human cytomegalovirus colocalize with and disrupt PML-associated nuclear bodies at very early times in infected permissive cells

The major immediate-early (MIE) gene products of human cytomegalovirus (HCMV) are nuclear phosphoproteins that are thought to play key roles in initiating lytic cycle gene regulation pathways. We have examined the intranuclear localization pattern of both the IE1 and IE2 proteins in virus-infected and DNA-transfected cells. When HCMV-infected human diploid fibroblast (HF) cells were stained with specific monoclonal antibodies, IE1 localized as a mixture of nuclear diffuse and punctate patterns at very early times (2 h) but changed to an exclusively nuclear diffuse pattern at later times. In contrast, IE2 was distributed predominantly in nuclear punctate structures continuously from 2 to at least 12 h after infection. These punctate structures resembled the preexisting PML-associated nuclear bodies (ND10 or PML oncogenic domains [PODs]) that are disrupted and dispersed by the IE110 protein as a very early event in herpes simplex virus (HSV) infection. However, HCMV differed from HSV by leading instead to a change in both the PML and SP100 protein distribution from punctate bodies to uniform diffuse patterns, a process that was complete in 50% of the cells at 2 h and in 90% of the cells by 4 h after infection. Confocal double-label indirect immunofluorescence assay analysis confirmed that both IE1 and IE2 colocalized transiently with PML in punctate bodies at very early times after infection. In transient expression assays, introduction of IE1-encoding plasmid DNA alone into Vero or HF cells produced the typical total redistribution of PML into a uniform nuclear diffuse pattern together with the IE1 protein, whereas introduction of IE2-encoding plasmid DNA alone resulted in stable colocalization of the IE2 protein with PML in the PODs. A truncated mutant form of IE1 gave large nuclear aggregates and failed to redistribute PML, and similarly a deleted mutant form of IE2 failed to colocalize with the punctate PML bodies, confirming the specificity of these effects. Furthermore, both Vero and U373 cell lines constitutively expressing IE1 also showed total PML relocalization together with the IE1 protein into a nuclear diffuse pattern, although a very small percentage of the cells which failed to express IE1 reverted to a punctate PML pattern. Finally, the PML redistribution activity of IE1 and the direct association of IE2 with PML punctate bodies were both confirmed by infection with E1A-negative recombinant adenovirus vectors expressing either IE1 or IE2 alone. These results confirm that transient colocalization with and disruption of PML-associated nuclear bodies by IE1 and continuous targeting to PML-associated nuclear bodies by IE2 are intrinsic properties of these two MIE regulatory proteins, which we suggest may represent critical initial events for efficient lytic cycle infection by HCMV.     

Sequence similarity between the 73-kilodalton protein of mammalian CPSF and a subunit of yeast polyadenylation factor I

The 3' ends of most eukaryotic messenger RNAs are generated by endonucleolytic cleavage and polyadenylation. In mammals, the cleavage and polyadenylation specificity factor (CPSF) plays a central role in both steps of the processing reaction. Here, the cloning of the 73-kilodalton subunit of CPSF is reported. Sequence analyses revealed that a yeast protein (Ysh1) was highly similar to the 73-kD polypeptide. Ysh1 constitutes a new subunit of polyadenylation factor I (PFI), which has a role in yeast pre-mRNA 3'-end formation. This finding was unexpected because in contrast to CPSF, PFI is only required for the polyadenylation reaction. These results contribute to the understanding of how 3'-end processing factors may have evolved.