Component of the packaging machinery which encapsidates the viral DNA into preformed capsids and transcriptional activator of the viral major late promoter (MLP). Binds, along with packaging proteins 2 and 3, to the specific packaging sequence on the left end of viral genomic DNA and displays ATPase activity thereby providing the power stroke of the packaging machinery. The activity of packaging protein IVa2 is stimulated by protein 33K which acts as a terminase. May be the protein that pumps DNA into the capsid powered by ATP hydrolysis. Specifically binds to the 5'-CG-3' nucleotides of the repeats making up the packaging sequence. Component of the DEF-A and DEF-B transcription factors that bind downstream elements of the major late promoter (MLP), and stimulate transcription from the MLP after initiation of viral DNA replication. DEF-A is a heterodimer packaging proteins 1 and 2 and DEF-B is a homodimer of packaging protein 1. Homodimer. Part of a genome packaging complex composed of packaging proteins 1, 2 and 3; this complex specifically binds to the packaging sequence on the left end of viral genomic DNA and performs packaging of the viral genome. Interacts with protein 33K. Located at a unique vertex of the capsid. Present in about 6-8 copies per virion. Expressed in the intermediate phase of the viral replicative cycle. Belongs to the adenoviridae packaging protein 1 family.