Xihe Zhang, Jing Shi, Xusheng Qiu, Qingqing Chai, Dylan A. Frabutt, Richard C. Schwartz, Yong-Hui Zheng
Online 1 May 2019 J. Virol. doi : 10.1128/JVI.00544-19
Serine incorporator 5 (SERINC5) is a recently identified restriction factor that strongly blocks HIV-1 entry but is counteracted by Nef. Notably, Tier 1 HIV-1 Env proteins are sensitive, whereas the majority of Tier 2/3 Env proteins are resistant to SERINC5, when viruses are produced from CD4-negative cells and tested by a single-round replication assay. Here, we investigated the Env-dependent SERINC5 antiviral mechanism by comparing Tier 1 NL Env with Tier 3 AD8 Env proteins. We found that when NL and AD8 viruses were inoculated into CD4+ T cells and human peripheral blood mononuclear cells (PBMCs), the propagation of both viruses was restricted to a similar level when Nef was not expressed. Using a bimolecular fluorescence complementation (BiFC) assay, we detected Env-Env association and Env-SERINC5 interaction. Much more NL Env-SERINC5 interaction was detected than was AD8 Env-SERINC5 interaction, which was further validated by immunoprecipitation. In addition, SERINC5 dissociated the NL Env trimeric complex more effectively than the AD8 Env trimeric complex, when CD4 was not expressed. However, when CD4 was expressed, Ser5 became more capable of interacting with AD8 Env and dissociating its trimeric complex. Moreover, AD8 and several other Tier 2/3 viruses produced in the presence of CD4 became sensitive to SERINC5 when measured by the single-round replication assay. Because Tier 1 and Tier 2/3 Env trimers have an open versus closed conformation, respectively, and CD4 opens the closed conformation, we conclude that SERINC5 selectively dissociates Env trimers with an open conformation to restrict HIV-1 replication.