Functional Mapping of Regions Involved in the Negative Imprinting of Virion Particle Infectivity and in Target Cell Protection by Interferon-Induced Transmembrane Protein 3 against HIV-1.

The interferon-induced transmembrane proteins (IFITMs) are a family of highly related antiviral factors that affect numerous viruses at two steps: in target cells by sequestering incoming viruses in endosomes and in producing cells by leading to the production of virions that package IFITMs and exhibit decreased infectivity. While most studies have focused on the former, little is known about the regulation of the negative imprinting of virion particle infectivity by IFITMs and about its relationship with target cell protection. Using a panel of IFITM3 mutants against HIV-1, we have explored these issues as well as others related to the biology of IFITM3, in particular virion packaging, stability, the relation to CD63/multivesicular bodies (MVBs), the modulation of cholesterol levels, and the relationship between negative imprinting of virions and target cell protection. The results that we have obtained exclude a role for cholesterol and indicate that CD63 accumulation does not directly relate to an antiviral behavior. We have defined regions that modulate the two antiviral properties of IFITM3 as well as novel domains that modulate protein stability and that, in so doing, influence the extent of its packaging into virions. The results that we have obtained, however, indicate that, even in the context of an IFITM-susceptible virus, IFITM3 packaging is not sufficient for negative imprinting. Finally, while most mutations concomitantly affect target cell protection and negative imprinting, a region in the C-terminal domain (CTD) exhibits a differential behavior, potentially highlighting the regulatory role that this domain may play in the two antiviral activities of IFITM3. IFITM proteins have been associated with the sequestration of incoming virions in endosomes (target cell protection) and with the production of virion particles that incorporate IFITMs and exhibit decreased infectivity (negative imprinting of virion infectivity). How the latter is regulated and whether these two antiviral properties are related remain unknown. By examining the behavior of a large panel of IFITM3 mutants against HIV-1, we determined that IFITM3 mutants are essentially packaged into virions proportionally to their intracellular levels of expression. However, even in the context of an IFITM-susceptible virus, IFITM3 packaging is not sufficient for the antiviral effects. Most mutations were found to concomitantly affect both antiviral properties of IFITM3, but one CTD mutant exhibited a divergent behavior, possibly highlighting a novel regulatory role for this domain. These findings thus advance our comprehension of how this class of broad antiviral restriction factors acts.