Involved, together with A1 protein, in the second step transfer (SST) which allows the completion of viral DNA into the host cell. Interacts with A1 protein; the two proteins form heterooligomers. Expressed pre-early in the viral replicative cycle. Expressed with a few other proteins from the first step transfer (FST) DNA. FST DNA corresponds to the first small portion of the genome that enters into the host cell at the beginning of infection before pausing. This gene is part of the long terminal repeats present at both ends of the viral genome and is thus duplicated. Belongs to the T5likevirus A2 protein family.