Highly Mutable Linker Regions Regulate HIV-1 Rev Function and Stability
The HIV-1 protein Rev is an essential viral regulatory protein that facilitates the nuclear export of intron-containing viral mRNAs. Its sequence is organized into short, structured, functionally well-characterized motifs joined by less understood linker regions. We recently carried out a competitive deep mutational scanning study, which determined the relative fitness of every amino acid at every position of Rev in replicating viruses. This study confirmed many known constraints in Revs established interaction motifs, but also identified positions of mutational plasticity within these regions as well as in surrounding linker regions. Here, we probe the mutational limits of these linkers by designing and testing the activities of multiple truncation and mass substitution mutations. We find that these regions possess previously unknown structural, functional or regulatory roles, not apparent from systematic point mutational approaches. Specifically, the N- and C-termini of Rev contribute to protein stability; mutations in a turn that connects the two main helices of Rev have different effects in nuclear export assays and viral replication assays; and a linker region which connects the second helix of Rev to its nuclear export sequence has structural requirements for function. Thus, we find that Rev function extends beyond its characterized motifs, and is in fact further tuned by determinants within seemingly plastic portions of its sequence. At the same time, Revs ability to tolerate many of these massive truncations and substitutions illustrates the overall mutational and functional robustness inherent in this viral protein.\n\nAuthor Summary (non-technical summary)HIV-1 Rev is an essential viral protein that controls a critical step in the HIV life cycle. It is responsible for transporting viral mRNA messages from the nucleus to the cytoplasm where they can contribute to the formation new virus particles. In order to understand how different regions of the Rev protein sequence are involved in its function, we introduced truncations and mass substitution mutations in the protein sequence and tested their effect on protein function. Through this study, we not only confirmed previous work highlighting known functionally important regions in Rev, but also found that a large portion of Rev, with little known functional roles influence Rev function and stability. We also show that although protein sequence is critical to its function, Rev can tolerate large variations to its sequence without disrupting its function significantly.