Pre/S Open Reading Frame Case Study

1.8.1 Pre-S/S Open Reading Frame (Pre-S/S ORF)
The pre-S/S ORF has three in-frame translation initiation codons that divide the gene into the pre-S1, pre-S2 and S regions which encodes the large (L), middle (M) and small (S) envelope proteins, respectively (Tiollais et al., 1985) . The S protein is 226 amino acids long, has a molecular mass of 25 kDa and shares DNA and amino acid sequence as the C-terminus of the M and L protein. The M protein has a 55-amino acid extension to the S protein at its N-terminus, giving it a molecular mass of 31 kDa, while the L protein a further 108-119-amino acid extention to the M protein at its N-terminus and a molecular mass of 43 kDa. Virions and subviral particles contain all three proteins
The first 19 amino acids of the pre-core region from a single sequence which directed the pre-core protein to the ER; this signal is eventually cleaved off by a host cell signal peptidase leaving the pre-core protein derivative, P22. The truncated pre-core protein is further modified by C- terminal cleavage of up to 34 amino acids as it passes through the ER and Golgi apparatus, resulting in a secreted polypeptide of 15-18 kDa, serologically defined as HBeAg (Ou et al., 1986 ; Standring, 1991). The HBeAg is thought to play a key role in inducing immune tolerance on an immature immune clearance of infected cells (Thomas et al., 1988; Milich et al., 1990a). Although, detection of HBeAg in the serum is a predictor of active viral replication (Okada et al., 1976), its biological role in viral life cycle remains yet to be elucidated (Milich and Liang, 2003). However, its absence does not rule out active infection as the emergence of pre-core mutants of HBV during the natural course of chronic infection can result in HBeAg negativity in the presence of hepatitis B viremia (Hadziyannis,
It covers nearly 80% of the genome and overlaps the other three ORFs. The product to the Pol gene is a multifunctional protein spanning approxmately 834 to 845 codons and comprises the DNA- and RNA- dependent DNA polymerase (reverse transcriptase, RT), RNase H and the terminal protein domains (Bartenschlager and Schaller, 1988; Radziwill et al., 1990) (Figure-1.6A). The terminal protein, or primase, is located at the N-terminus of the polymerase and separated from the DNA polymerase domain by a non-essential spacer region. It is covalently bound to the 5’ end of the minus DNA strand and is involved in protien priming during reverse transcription (Wang and Seeger, 1992). The C-terminus portion of the polypeptide is responsible for genome replication, comprising of the viral polymerase which is involved in reverse transcription of pregenomic RNA (pgRNA) to minus strand and DNA-dependent-DNA polymerse to form the plus strand. There is recent evidence demonstrating the presence of pyrophosphorolytic activity in DHBV whereby incorporated dNTPs are removed from the elongating DNA by the reverse process of DNA polymerisation: DNAn + dNTP Ppi (Urban et al., 2001). It is possible that the ability of DHBV RT to remove recently incorporated dNTP by pyrophosphorolysis during viral replication may be a way to remove incorrectly incorporated dNTP, and thus function as a surrogate 3`-5` exonuclease activity. Finally,