NUCLEOPLASMIC AND NUCLEOLAR DISTRIBUTION OF THE ADENOVIRUS IVA2 GENE-PRODUCT

Sequence elements (DE) located downstream of the adenovirus major late promoter start site have previously been shown to be essential for th e activation of this promoter after the onset of viral DNA replication , Two proteins (DEF-A and DEF-B) bind to these elements in a late-phas e-dependent manner and contribute to this activation, DEF-B correspond s to a dimer of the adenovirus IVa2 gene product (pIVa2, 449 residues) , while DEF-A is a heteromeric protein also comprising pIVa2, As revea led by specific immunofluorescence staining of infected cells, pIVa2 i s targeted to the nucleus, where it distributes to both nucleoplasmic and nucleolar structures. We have identified the pIVa2 nuclear localiz ation signal (NLS) as a basic peptide element at the C terminus of the protein (residues 432 to 449), An element essential for nucleolar loc alization (NuLS) has been mapped in the N-terminal part of pIVa2 (betw een residues 50 and 136), While NuLS activity is dependent upon an int act NLS, we show that both NLS and NuLS functions are independent of s pecific DNA-binding activity. As visualized by immunoelectron microsco py, pIVa2 is detected in the nucleoplasm at the level of the fibrillog ranular network which is active in viral transcription, More surprisin gly, pIVa2 accumulates within electron-dense amorphous inclusions foun d both in the nucleoplasm and in the nucleolus, Altogether, these resu lts suggest that, besides controlling major late promoter transcriptio n, pIVa2 serves additional, as yet unknown functions.