Abstract
In most cases, proteolytic processing of the retroviral Pol portion of the Gag-Pol polyprotein precursor produces protease (PR), reverse transcriptase (RT), and integrase (IN). However, foamy viruses (FVs) express Pol separately from Gag and, when Pol is processed, only the IN domain is released. Here, we report a 2.9 Å resolution crystal structure of the mature PR-RT from prototype FV (PFV) that can carry out both proteolytic processing and reverse transcription but is in a configuration not competent for proteolytic or polymerase activity. PFV PR-RT is monomeric and the architecture of PFV PR is similar to one of the subunits of HIV-1 PR, which is a dimer. There is a C-terminal extension of PFV PR (101-145) that consists of two helices which are adjacent to the base of the RT palm subdomain, and anchors PR to RT. The polymerase domain of PFV RT consists of fingers, palm, thumb, and connection subdomains whose spatial arrangements are similar to the p51 subunit of HIV-1 RT. The RNase H and polymerase domains of PFV RT are connected by flexible linkers. Significant spatial and conformational (sub)domain rearrangements are therefore required for nucleic acid binding. The structure of PFV PR-RT provides insights into the conformational maturation of retroviral Pol polyproteins.
Highlights
Foamy viruses (FVs), the most ancient retroviruses, have the same genome organization as retrotransposons and the architecture of their structural and enzymatic proteins is similar [1,2,3,4]
We made the C280S mutation to improve the solution behavior of the protein akin to HIV-1 reverse transcriptase (RT) C280S, which is known to improve the behavior of purified HIV-1 RT
In the case of prototype FV (PFV) proteasereverse transcriptase fusion (PR-RT), both activities must occur in the mature polyprotein form
Summary
Foamy viruses (FVs), the most ancient retroviruses, have the same genome organization as retrotransposons and the architecture of their structural and enzymatic proteins is similar [1,2,3,4]. Many RNA viruses, retroviruses, and retrotransposons synthesize polyproteins that are proteolytically processed by cognate viral protease(s) into their functional mature forms [6,7,8,9]. Of the proteins encoded in the retroviral genome, reverse transcriptases (RTs) and proteases (PRs) are the most conserved structurally and functionally [10,11,12], despite having low sequence identity. Lentiviruses, such as HIV-1, synthesize their enzymatic proteins as components of Gag-Pol polyprotein fusions. The mature PR-RT has all of the enzymatic functions of PR and RT [reviewed in [15]]
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