Avian polyomavirus agnoprotein 1a is incorporated into the virus particle as a fourth structural protein, VP4

Agnoproteins, encoded by the 5'-region of the late bicistronic mRNA of some polyomaviruses, are small proteins with largely unknown functions. In avia n polyomavirus (APV)-infected cells, mRNAs of seven putative agnoproteins h ave been observed, Recently, it has been shown that agnoprotein la and its truncated variant agnoprotein Ib, encoded by the predominant mRNA species, are essential for APV replication. Here, the presence of agnoprotein 1 a is demonstrated in the nucleus of APV-infected cells and in purified APV part icles. Interaction between agnoprotein la and the major structural protein, VP1, was demonstrated by co-immunoprecipitation experiments using lysates of recombinant baculovirus-infected insect cells. With proteins expressed i n E, coli, binding to double-stranded DNA in a sequence-unspecific manner w as shown for agnoprotein 1 a, whereas agnoprotein Ib failed to bind. A leuc ine zipper-like motif present in agnoprotein la is considered to be involve d in DNA binding, Due to the absence of any structural or functional homolo gies between APV agnoprotein la and the agnoproteins of mammalian polyomavi ruses, it is suggested that this protein should be renamed VP4, indicating its function as a fourth structural protein of APV.