Adenoviruses encode a cysteme endopeptidase synthesized late in virus infection which is essential for virlon maturation and infectivity (1,2). The enzyme 1s encapsidated (approx 20 molecules per vinon) and may also have a role during decapsidation (3, 4, 5). Although there are approx 100 adenovirus serotypes known to infect vertebrates, so far only the human adenovirus type 2 (Ad2) enzyme has been studied. The recombinant protein expressed in Escherichia coli and insect cells has been purified and characterized (3,6, 7, 8). The enzyme is a 204-residue monomer of 24,838 Dalton with a pI of 10.59 and optimal activity at 45°C and pH 8.0. The recombinant enzyme is stimulated by an 11-amino acid cleavage fragment from viral protein pre-VI. GVQSLKRRRCF. The peptide is presumed to regulate enzyme activity in vivo during virus infection. It is bound to C104 on the enzyme via a disulphide bridge (9). Mutational analysis and X-ray chrystallography identified the active-site triad as H54-C122-E71 (8, 9, 10, 11). The substrate specificity of the enzyme is (M,I,L)XGG-X or (M,I,L)XGX-G (12). Alkylating agents and E64 inhibit the protease and virus infection as expected (13). Specific inhibitors are not yet available. To date, 17 protease genes have been sequenced in different virus serotypes. The translated amino acid sequences range from 201 to 214 residues and show both variable and highly conserved regions (1,11).
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