Involved in the degradation of host DNA and the shutoff of host genes and phage pre-early genes. Together with A2 protein, involved in the second step transfer (SST) of viral DNA into the host cell, which leads to the complete entry of viral DNA into the host cell. Homooligomer (PubMed:5136582). Interacts with A2 protein; the two proteins form heterooligomers (PubMed:5136582). Part of a complex containing the host RNA polymerase, protein A1 as well as another viral protein of 11 kDa; these interactions are probably involved in the transcription shutoff of pre-early genes (PubMed:7033565). 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.