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Hepatitis A virus (HAV) encodes a single polyprotein which is posttranslationally processed into the functional structural and nonstructural proteins. Only one protease, viral protease 3C, has been implicated in the nine protein scissions. Processing of the capsid protein precursor region generates a unique intermediate, PX (VP1-2A), which accumulates in infected cells and is assumed to serve as precursor to VP1 found in virions, although the details of this reaction have not been determined. Coexpression in transfected cells of a variety of P1 precursor proteins with viral protease 3C demonstrated efficient production of PX, as well as VP0 and VP3; however, no mature VP1 protein was detected. To identify the C-terminal amino acid residue of HAV VP1, we performed peptide sequence analysis by protease-catalyzed [18O]H2O incorporation followed by liquid chromatography ion-trap microspray tandem mass spectrometry of HAV VP1 isolated from purified virions. Two different cell culture-adapted isolates of HAV, strains HM175pE and HM175p35, were used for these analyses. VP1 preparations from both virus isolates contained heterogeneous C termini. The predominant C-terminal amino acid in both virus preparations was VP1-Ser274, which is located N terminal to a methionine residue in VP1-2A. In addition, the analysis of HM175pE recovered smaller amounts of amino acids VP1-Glu273 and VP1-Thr272. In the case of HM175p35, which contains valine at amino acid position VP1-273, VP1-Thr272 was found in addition to VP1-Ser274. The data suggest that HAV 3C is not the protease responsible for generation of the VP1 C terminus. We propose the involvement of host cell protease(s) in the production of HAV VP1.

The 2A gene of hepatitis A virus (HAV) bears no obvious similarity to the corresponding genes of other picornaviruses and has no known function. In a preliminary effort to gain information about the HAV 2A gene product, we constructed several HAV cDNAs containing deletions of 30 or 45 nucleotides in the predicted central portion of the 2A gene. These deletions did not affect the sites of protein processing, although the rates or efficiencies of polyprotein cleavage at the surrounding cleavage junctions appeared slightly reduced. Transfection of FRhK-4 cells with RNA transcripts of the deleted HAV cDNAs generated small foci of infected cells and produced infectious virus that retained the deletion mutations. In contrast, a single amino acid insertion in the 2B coding region was lethal to virus replication despite normal protein processing. Another deletion, which included the predicted 2A/2B junction and extended into the 2B coding sequence, did not support polyprotein processing or generate viable virus. One of the viable internal 2A deletions was introduced into a wild-type HAV cDNA background, and transcripts were tested for infectivity by inoculation directly into the livers of two marmosets. Both animals seroconverted, displayed elevated serum liver enzymes, and excreted infectious virus. Thus, deletion of 10 or 15 amino acid residues from the predicted central portion of the 2A protein was tolerated with only relatively minor effects on the growth of HAV in cultured cells and in marmoset liver.

To determine the P3 region protein-processing sites cleaved by the hepatitis A virus 3C protease, a nested set of constructs containing a portion of 3A (3A* [the asterisk denotes an incomplete protein]), 3B and 3C and various amounts of 3D, fused in frame to Escherichia coli TrpE-coding sequences under control of the tryptophan promoter, was made. Additional plasmids that encoded a portion of 2C (2C*) and the P3 proteins, including complete or incomplete 3D sequences, were constructed. After induction, E. coli containing these recombinant plasmids produced high levels of fusion proteins as insoluble aggregates. 3C-mediated cleavage products were identified by comparison of expression with a matching set of plasmids, containing an engineered mutation in 3C. Cleavage products were detected by immunoblot analyses by using antisera against the TrpE protein, against 3D*, and against 3CD*. Scissile bonds were determined by N-terminal amino acid sequencing of the proteins formed by cleavage. The results showed that when a portion of 2C was present, the primary cleavage by the 3C protease was between 2C and 3A, and the cleavage site was QG, as predicted by J. I. Cohen, J. R. Ticehurst, R. H. Purcell, A. Buckler-White, and B. M. Baroudy, J. Virol. 61:50-59, 1987. Very little further cleavage of the released P3 protein was detected. When the fusion protein contained no 2C and included only 3A*-to-3D sequences, efficient cleavage occurred between 3B and 3C, at the QS pair, also as predicted by Cohen et al. (J. Virol. 61:50-59, 1987). The latter proteins were also cleaved between 3C and 3D, but less efficiently than between 3B and 3C. Extracts of bacteria expressing proteins from 3A* to 3D also cleaved a radiolabelled hepatitis A virus substrate containing VP1*2ABC* sequences in trans.

A series of plasmids containing hepatitis A virus (HAV) cDNA was constructed such that positive-strand HAV RNA could be transcribed with T7 RNA polymerase. The plasmids differed in the number of 5'-terminal nucleotides representing the junctions between vectors and HAV sequences that were present in the transcripts. When these transcripts were used to transfect cultured BS-C-1 cells, it was found that only those transcripts that contained all of the 5'-terminal HAV nucleotides, in addition to one or more nucleotides from the vector, were capable of initiating an infectious cycle leading to production of progeny virus. Transcripts that contained one 5'-terminal nucleotide from the vector sequence but were missing two uridylate residues corresponding to the first two nucleotides of HAV sequences, or were missing U and C residues corresponding to nucleotides 2 and 3 of the HAV sequence, were not infectious. A similar plasmid containing poliovirus cDNA was engineered to produce transcripts similarly lacking the first two uridylate residues of the poliovirus RNA sequence. These transcripts were infectious.