The adenovirus type 5 E1b 55K and E4 Orf3 proteins associate in infected cells and affect ND10 components

Three early proteins expressed by adenovirus type 5, E1b 55K, E4 OrF3 and E 4 Orf6, are involved in regulating late viral gene expression. It has previ ously been shown that 55K associates with Orf6, Here we show that 55K also associates with Orf3 and that this interaction is necessary for 55K to loca lize to the nuclear matrix fraction of the cell. From our data, we infer th at the OrF3 and Orf6 interactions with 55K may be mutually exclusive. The O rf3 protein is also known to associate with and cause the reorganization of cell nucleus structures known as ND10 or PODs. Consistent with the observe d increase in the biochemical interaction between 55K and Orf3 in the absen ce of Orf6, the 55K association with Orf3 in ND10 was also found to increas e in the absence of Orf6. The most studied cellular component of ND10 is PM L, a complex protein present in a range of isoforms, some of which are modi fied by conjugation to the small ubiquitin-like protein PIC-1. The pattern of PML isoforms was altered in adenovirus-infected cells, in that a number of additional isoform bands appeared in an Orf3-dependent manner, one of wh ich became predominant later in infection. As for ND10 reorganization, neit her Orf6 nor 55K was required for this effect. Therefore it is likely that these changes in PML are related to the changes in ND10 structure that occu r during infection.