Adenovirus endopeptidase hydrolyses human squamous cell carcinoma antigens in vitro but not ex vivo

Adenain, the adenovirus endoprotease (a review)

With the possible exception of very simple viruses, most viruses appear to encode at least one virus specific endopeptidase. In addition to facilitating the orchestrated fragmentation of polyproteins of RNA viruses, these proteolytic enzymes may also be involved in the suppression of host protein synthesis, the regulation of virus assembly, the egress and subsequent uncoating in another cycle of infection of both RNA and DNA viruses. The endopeptidase encoded by adenoviruses (AVP or adenain) appears to be involved in several of these functions. Most of the literature concerns the protease of human adenovirus type 2, but there are good reasons to believe that the proteases of other adenovirus serotypes will be very similar. For a review see Weber [1,2].

Substrate specificity of adenovirus protease

The adenovirus protease, adenain is functionally required for virion uncoating and virion maturation and release from the infected cell. In addition to hydrolysis of precursor proteins at specific consensus sites, adenain has also been observed to cleave viral proteins at other sites. Here we re-examine the sequences in the consensus sites and also the phenomena of cleavage at other sites on viral proteins II, 100K, V, VI and VII. An examination of the eight residues flanking the scissile bond in 274 consensus sites from 36 different adenovirus serotypes in the DNA sequence databanks provided the following main conclusions: (1) two types of consensus sites, type 1, (M,I,L)XGX-G and type 2, (M,I,L)XGG-X, (2) the variant positions P(3) and P(1) never contained C,P,D,H,W,Y and C,P,G,M amino acids, respectively in type 1, (3) the variant positions P(3) and P(1)' never contained C,D,L,W and C,P,D,Q,H,Y,W amino acids, respectively in type 2, and (4) the thiol forming C residue occurred only twice within the eight residues flanking the scissile bond and that in the P(4)' position. Six unusual serotypes had (M,L,I)XAT-G as the PVII consensus site. Adenain has been proposed to cleave protein VI at an unknown site in the course of virion uncoating. The cleavage of capsid protein VI in the absence of a consensus site is confirmed here in vitro using recombinant adenain. Virion proteins II, V and VII and the nonstructural protein 100K were also digested in vitro into discrete fragments by recombinant adenain. We conclude that adenain preferentially cleaves viral proteins at their consensus sites, but is capable, in vitro of cleavages at other discrete sites which resemble the consensus cleavage sites.

Interactions of human lacrimal and salivary cystatins with adenovirus endopeptidase

Over 100 serotypes of adenoviruses have been implicated in a variety of human and domesticated animal pathologies and some serotypes are widely used as gene transfer vectors. Aside from the limited use of vaccines for specific serotypes, little effort has been expended in the development of antivirals. The objective here was to study the effect of cystatins from human saliva (CS) and tears (CT), two points of viral entry, on adenain, the adenovirus type 2 encoded proteinase, which is absolutely required for infectivity. Two molecular weight species (13 and 14.5 kDa) were purified from both fluids at a yield of 5 mg/l. In vitro adenain activity was inhibited to 50% at a molar ratio of 5 CS:1 adenain and 3 CT:1 adenain. By comparison, papain was inhibited to 50% at a molar ratio of 2 CS:1 papain and 1.5 CT:1 papain. Adenain differed from papain in response to CS and chicken egg white (CEW) cystatin in being stimulated at low concentrations, and in being inhibited only at very high concentrations of cystatins. The presence of cleavage consensus sites specific to adenain in the human cystatins could drive the adenain-cystatin interaction predominantly in the substrate pathway direction. However, we found that the cystatins could only be digested after denaturation and by highly active fresh enzyme preparations. Our experiments designed to test the nature of the interaction between adenain and cystatins suggest a docking model for the adenain-human cystatin interaction, similar to that proposed for papain and CEW. At equilibrium the dissociation constant, K(d), between adenain and CT was 1.2 nM. The kinetic parameters determined here suggest a simple reversible mechanism for the inhibition of adenain by human cystatins. We conclude that the cystatins present in tears and saliva are unlikely to play a significant role in inhibiting adenovirus infections.