Department of Pathogenic Microbiology and Immunology, School of Basic Medical Sciences, Xi'an Jiaotong University Health Science Center, Xi'an, People's Republic of China
Division of Gastroenterology, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, Michigan, USA.
J Virol. 2019 Oct 29;93(22). doi: 10.1128/JVI.00742-19. Print 2019 Nov 15.
The endoplasmic reticulum (ER)-resident proteins vesicle-associated membrane protein (VAMP)-associated protein A and B (VAPA and VAPB) have been reported to be necessary for efficient hepatitis C virus (HCV) replication, but the specific mechanisms are not well understood. VAPs are known to recruit lipid transfer proteins to the ER, including oxysterol binding protein (OSBP), which has been previously shown to be necessary for cholesterol delivery to the HCV replication organelle in exchange for phosphatidylinositol 4-phosphate [PI(4)P]. Here, we show that VAPA and VAPB are redundant for HCV infection and that dimerization is not required for their function. In addition, we identify the phosphatidylinositol transfer protein Nir2 as an effector of VAPs to support HCV replication. We propose that Nir2 functions to replenish phosphoinositides at the HCV replication organelle to maintain elevated steady-state levels of PI(4)P, which is removed by OSBP. Thus, Nir2, along with VAPs, OSBP, and the phosphatidylinositol 4-kinase, completes a cycle of phosphoinositide flow between the ER and viral replication organelles to drive ongoing viral replication. Hepatitis C virus (HCV) is known for its ability to modulate phosphoinositide signaling pathways for its replication. Elevated levels of phosphatidylinositol 4-phosphate [PI(4)P] in HCV replication organelles (ROs) recruits lipid transfer proteins (LTPs), like oxysterol-binding protein (OSBP). OSBP exchanges PI(4)P with cholesterol, thus removing PI(4)P from the HCV RO. Here, we found that the phosphatidylinositol transfer protein Nir2 acts as an LTP and may replenish PI at the HCV RO by interacting with VAMP-associated proteins (VAPs), enabling continuous viral replication during chronic infection. Therefore, the coordination of OSBP, Nir2, and VAPs completes our understanding of the phosphoinositide cycle between the ER and HCV ROs.
内质网（ER）驻留蛋白囊泡相关膜蛋白（VAMP）相关蛋白 A 和 B（VAPA 和 VAPB）已被报道对于丙型肝炎病毒（HCV）的有效复制是必要的，但具体机制尚不清楚。VAPs 已知可将脂质转运蛋白募集到 ER，包括已被证明对于胆固醇向 HCV 复制细胞器的递呈是必要的甾醇结合蛋白（OSBP），以换取磷脂酰肌醇 4-磷酸 [PI(4)P]。在这里，我们表明 VAPA 和 VAPB 对于 HCV 感染是冗余的，并且其功能不需要二聚化。此外，我们确定了磷酸肌醇转移蛋白 Nir2 是 VAPs 的效应物，以支持 HCV 复制。我们提出 Nir2 的功能是在 HCV 复制细胞器处补充磷酸肌醇，以维持 PI(4)P 的稳定状态水平升高，OSBP 会将其去除。因此，Nir2 与 VAPs、OSBP 和磷脂酰肌醇 4-激酶一起，完成了 ER 和病毒复制细胞器之间的磷酸肌醇流循环，以驱动持续的病毒复制。丙型肝炎病毒（HCV）以其调节磷酸肌醇信号通路进行复制的能力而闻名。在 HCV 复制细胞器 (RO) 中，磷酸肌醇 4-磷酸 [PI(4)P] 的水平升高会募集脂质转运蛋白（LTP），如甾醇结合蛋白（OSBP）。OSBP 将 PI(4)P 与胆固醇交换，从而将 PI(4)P 从 HCV RO 中去除。在这里，我们发现磷酸肌醇转移蛋白 Nir2 作为 LTP 发挥作用，并且可以通过与囊泡相关蛋白（VAPs）相互作用来补充 HCV RO 中的 PI，从而在慢性感染期间实现持续的病毒复制。因此，OSBP、Nir2 和 VAPs 的协调完成了我们对 ER 和 HCV ROs 之间磷酸肌醇循环的理解。
