Hepatitis B virus DNA polymerase


Hepatitis B virus DNA polymerase is a hepatitis B viral protein. It is a DNA polymerase that can use either DNA or RNA templates and a ribonuclease H that cuts RNA in the duplex. Both functions are supplied by the reverse transcriptase domain.

Structure

The hepadnaviral P protein is organized into three domains: an N-terminal domain covering the terminal and the spacer, an RT domain related to every other RT domain, and a C-terminal domain regulating the RNase H activity.

Function

In HBV the pgRNA, is translated into the viral polymerase and core proteins, and then encapsidated and reverse transcribed to form a new rcDNA molecule. In this stage, the HBV polymerase has some tasks and functions. The HBV polymerase is a multifunctional enzyme with RNA-dependent and DNA-dependent polymerase and RNase H activities. Furthermore, hepadnavirus polymerases contain a terminal protein domain that contains a tyrosine residue that serves as a primer for the synthesis of the DNA strand.
In reverse transcription steps, existing of HBV core protein dimers is required for packaging of the pgRNA/polymerase complex. Then, after viral polymerase binds to the packaging signal found at the 5′ end of the pgRNA, they are incorporated into the viral capsid. Inside the capsid, the pgRNA undergoes reverse transcription, which is initiated by protein priming at the tyrosine residue of the HBV polymerase. Thus, DNA strand is made. At the same time, degradation of the RNA template is took place by the RNase H activity of the polymerase. A short RNA of about 15–18 nucleotides at the 5′ end of the pgRNA is not degraded and it is used as primer for DNA strand synthesis.
The resulting RC-DNA is partially double stranded. The DNA strand is longer than a genome length, with a covalently bound polymerase and a redundant flap at the 5′ end. However, the DNA strand synthesis is uncompleted by the polymerase, and there is a gap exists down to the 3′ end of the DNA strand.