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 1 and 3, to the specific packaging sequence on the left end of viral genomic DNA and plays an active role in packaging of the viral genome into preformed capsids. Specifically binds to the 5'-TTTG-3' nucleotides of the repeats making up the packaging sequence. Forms a transcription factor called DEF-A through cooperative binding with packaging protein 1 (Probable). DEF-A binds to downstream elements of the major late promoter (MLP) and stimulates transcription from the MLP after initiation of viral DNA replication. Simultaneously suppresses early gene expression and is thus likely to participate in the early-late switch in the expression pattern of the late viral proteins. May as well enhance transcription from IVa2 and pIX promoters. 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. Self-assembles into higher-order structures. Expressed at the late phase of the viral replicative cycle. Probably already expressed from the early to late transition. All late proteins expressed from the major late promoter are produced by alternative splicing and alternative polyadenylation of the same gene giving rise to non-overlapping ORFs. Expression of packaging protein 2 and splicing factor is controlled by a L4 promoter distinct from the major late promoter. Unspliced isoform. Belongs to the adenoviridae splicing factor family.