Requirement of heat shock protein 90 for human hepatitis B virus reverse transcriptase function

The initiation of reverse transcription and nucleocapsid assembly in hepatitis B virus (HBV) depends on the specific recognition of an RNA signal (the packaging signal, ε) on the pregenomic RNA (pgRNA) by the viral reverse transcriptase (RT). RT-ε interaction in the duck hepatitis B virus (DHBV) was recently shown to require the molecular chaperone complex, the heat shock protein 90 (Hsp90). However, the requirement for RT-ε interaction in the human HBV has remained unknown due to the inability to obtain a purified RT protein active in specific ε binding. We now report that Hsp90 is also required for HBV RT-ε interaction. Inhibition of Hsp90 led to diminished HBV pgRNA packaging into nucleocapsids in cells, which depends on RT-ε interaction. Furthermore, using truncated HBV RT proteins purified from bacteria and five purified Hsp90 chaperone factors, we have developed an in vitro RT-ε binding assay. Our results demonstrate that Hsp90, in a dynamic process that was dependent on ATP hydrolysis, facilitated RT-ε interaction in HBV, as in DHBV. Specific ε binding required sequences from both the amino-terminal terminal protein and the carboxy-terminal RT domain. Only the cognate HBV ε but not the DHBV ε, could bind the HBV RT proteins. Furthermore, the internal bulge, but not the apical loop, of ε was required for RT binding. The establishment of a defined in vitro reconstitution system has now paved the way for future biochemical and structural studies to elucidate the mechanisms of RT-ε interaction and chaperone activation.