80α is a temperate, double-stranded DNA bacteriophage of Staphylococcus aureus that can act as a "helper" for the mobilization of S. aureus pathogenicity islands (SaPIs), including SaPI1. When SaPI1 is mobilized by 80α, the SaPI genomes are packaged into capsids that are composed of phage proteins, but that are smaller than those normally formed by the phage. This size determination is dependent on SaPI1 proteins CpmA and CpmB. Here, we show that co-expression of the 80α capsid and scaffolding proteins in S. aureus, but not in E. coli, leads to the formation of procapsid-related structures, suggesting that a host co-factor is required for assembly. The capsid and scaffolding proteins also undergo normal N-terminal processing upon expression in S. aureus, implicating a host protease. We also find that SaPI1 proteins CpmA and CpmB promote the formation of small capsids upon co-expression with 80α capsid and scaffolding proteins in S. aureus.