Processing of the l1 52/55k protein by the adenovirus protease: a new substrate and new insights into virion maturation.

Late in adenovirus assembly, the viral protease (AVP) becomes activated and cleaves multiple copies of three capsid and three core proteins. Proteolytic maturation is an absolute requirement to render the viral particle infectious. We show here that the L1 52/55k protein, which is present in empty capsids but not in mature virions and is required for genome packaging, is the seventh substrate for AVP. A new estimate on its copy number indicates that there are about 50 molecules of the L1 52/55k protein in the immature virus particle. Using a quasi-in vivo situation, i.e., the addition of recombinant AVP to mildly disrupted immature virus particles, we show that cleavage of L1 52/55k is DNA dependent, as is the cleavage of the other viral precursor proteins, and occurs at multiple sites, many not conforming to AVP consensus cleavage sites. Proteolytic processing of L1 52/55k disrupts its interactions with other capsid and core proteins, providing a mechanism for its removal during viral maturation. Our results support a model in which the role of L1 52/55k protein during assembly consists in tethering the viral core to the icosahedral shell and in which maturation proceeds simultaneously with packaging, before the viral particle is sealed.