The human adenovirus type 5 E4 transcription unit has the potential to
encode at least seven distinct polypeptides from reading frames acces
sed by differential splicing of a single primary transcript. Only some
of these polypeptides have yet been detected during viral infection o
f cultured cells. Mutational inactivation of the reading frames whose
products have not been described has no apparent effect on the growth
of virus in standard cultured human cell lines, indicating that these
proteins, if they exist, have only a subtle, non-essential role in the
replication cycle. We have raised an antiserum to one of these undefi
ned products, E4 Orf2, expressed in bacteria. Using this reagent, it w
as possible to show that Orf2 was expressed during the lytic cycle in
HeLa cells, being a soluble cytoplasmic component appearing with early
kinetics. No association of Orf2 protein with other infected cell com
ponents was detected.