Super-Resolution Fluorescence Imaging Reveals That Serine Incorporator Protein 5 Inhibits Human Immunodeficiency Virus Fusion by Disrupting Envelope Glycoprotein Clusters

Abstract

Serine incorporator protein 5 (SERINC5) is the host antiretroviral factor that reduces HIV-1 infectivity by incorporating into virions and inhibiting the envelope glycoprotein (Env) mediated virus fusion with target cells. We and others have shown that SERINC5 incorporation into virions alters the Env structure and sensitizes the virus to broadly neutralizing antibodies targeting cryptic Env epitopes. We have also found that SERINC5 accelerates the loss of Env function over time compared to control viruses. However, the exact mechanism by which SERINC5 inhibits HIV-1 fusion is not understood. Here, we utilized 2D and 3D super-resolution microscopy to examine the effect of SERINC5 on the distribution of Env glycoproteins on single HIV-1 particles. We find that, in agreement with a previous report, Env glycoproteins form clusters on the surface of mature virions. Importantly, incorporation of SERINC5, but not SERINC2, which lacks antiviral activity, disrupted Env clusters without affecting the overall Env content. We also show that SERINC5 and SERINC2 also form clusters on single virions. Unexpectedly, Env and SERINC molecules exhibited poor codistribution on virions, as evidenced by much greater Env–SERINC pairwise distances compared to Env–Env distances. This observation is inconsistent with the previously reported interaction between Env and SERINC5 and suggests an indirect effect of SERINC5 on Env cluster formation. Collectively, our results reveal a multifaceted mechanism of SERINC5-mediated restriction of HIV-1 fusion that, aside from the effects on individual Env trimers, involves disruption of Env clusters, which likely serve as sites of viral fusion with target cells.

Cholesterol in the Viral Membrane is a Molecular Switch Governing HIV‐1 Env Clustering

“…Based on the observation that SERINC5 did not codistribute with Env the authors conclude that its effect in clustering is indirect, and hypothesize that its effect could be based on sequestering of viral membrane chol by SERINC5. [ 62 ] Our results directly support the hypothesis that sequestering of chol by SERINC5 would remove the molecular switch driving Env redistribution and hamper entry.…”

Variation in HIV-1 Nef function within and among viral subtypes reveals genetically separable antagonism of SERINC3 and SERINC5

“…As a result, most reports focus on Nef's ability to antagonize SERINC5, and its impact on SERINC3 remains relatively understudied. While the mechanism of SERINC's antiviral activity has not been fully clarified, a recent study found that SERINC5 disrupts viral Envelope (Env) clusters on the virion surface through an indirect process [18]; however, other studies suggest that a direct interaction between Env and SERINC5 may result in conformational changes to Env that impair its function [19,20]. Regardless of the precise mechanism(s) of action, by removing SERINC proteins from the infected cell surface, Nef reduces their incorporation into virions and counteracts their antiviral function [7,8].…”

HIV-cell membrane fusion intermediates are restricted by Serincs as revealed by cryo-electron and TIRF microscopy

“…Whereas isoforms Serinc3 and Serinc5 can block infection of subsequent cells, other isoforms, including Serinc2, do not. Previous work has suggested that Serincs may act at the very earliest steps of membrane fusion by changing the conformation or clustering of unliganded Env (14), as evidenced by increased binding of broadly neutralizing antibodies to Env on Serinc5-containing viral particles (15)(16)(17). Serincs are integral membrane proteins that have been reported to be involved in lipid synthesis (12).…”