Chaperone activation of the hepadnaviral reverse transcriptase for template RNA binding is established by the Hsp70 and stimulated by the Hsp90 system.

Hepadnaviruses are DNA viruses that replicate by protein-primed reverse transcription, employing a specialized reverse transcriptase (RT), P protein. DNA synthesis from the pregenomic RNA is initiated by binding of P to the epsilon signal. Using epsilon as template and a Tyr-residue for initiation, the RT synthesizes a DNA oligo (priming) as primer for full-length DNA. Priming strictly requires prior RT activation by chaperones. Active P-epsilon complexes have been reconstituted in vitro, but whether in addition to the heat-shock protein 70 (Hsp70) system the Hsp90 system is essential has been controversial. Here we quantitatively compared Hsp70 versus Hsp70 plus Hsp90 RT activation, and corroborated that the Hsp70 system alone is sufficient; however, Hsp90 as well the Hsp70 nucleotide exchange factor Bag-1 markedly stimulated activation by increasing the steady-state concentration of the activated metastable RT form P*, though by different mechanisms. Hsp90 inhibition in intact cells by geldanamycin analogs blocked hepadnavirus replication, however not completely and only at severely cytotoxic inhibitor concentrations. While compatible with a basal level of Hsp90 independent in vivo replication, unambiguous statements are precluded by the simultaneous massive upregulation of Hsp70 and Hsp90.