Chromatin assembly factor 1 contains three subunits, p150, p60, and p48. It is essential for coupling nucleosome assembly to newly synthesized DNA. Whether chromatin assembly factor 1 subunits have functions beyond escorting histones, which depends on the complex formation of p150 and p60, has been an issue of great interest. This study reveals a novel role of p150, but not p60, in gene-specific transcriptional activation. We found that p150 transcriptionally activated an essential viral promoter, the major immediate early promoter (MIEP) of the human cytomegalovirus, independently of p60. Knocking down p150 decreased the MIEP function in both transfected and virally infected cells. The chromatin immunoprecipitation analysis and the in vitro protein-DNA binding assay demonstrated that p150 used its KER domain to associate with the MIEP from ؊593 to ؊574 bp. The N-terminal 244 residues were also found essential for p150-mediated MIEP activation, likely through recruiting the acetyltransferase p300 to acetylate local histones. Domain swapping experiments further showed that the KER and the N terminus of p150 acted as an independent DNA binding and transcriptional activation domain, respectively. Because p60 did not seem involved in the reaction, together these results indicate for the first time that p150 directly activates transcription, independently of its histone deposition function.
Chromatin assembly factor 1 (CAF1) consisting of p150, p60 and p48 is known to assemble histones onto newly synthesized DNA and thus maintain the chromatin structure. Here, we show that CAF1 expression was induced in human cytomegalovirus (HCMV)-infected cells, concomitantly with global chromatin decondensation. This apparent conflict was thought to result, in part, from CAF1 mislocalization to compartments of HCMV DNA synthesis through binding of its largest subunit p150 to viral immediate-early protein 2 (IE2). p150 interaction with p60 and IE2 facilitated HCMV DNA synthesis. The IE2Q548R mutation, previously reported to result in impaired HCMV growth with unknown mechanism, disrupted IE2/p150 and IE2/histones association in our study. Moreover, IE2 interaction with histones partly depends on p150, and the HCMV-induced chromatin decondensation was reduced in cells ectopically expressing the p150 mutant defective in IE2 binding. These results not only indicate that CAF1 was hijacked by IE2 to facilitate the replication of the HCMV genome, suggesting chromatin assembly plays an important role in herpesviral DNA synthesis, but also provide a model of the virus-induced chromatin instability through CAF1.
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