Vaccinia viral A26 protein is a fusion suppressor of mature virus and triggers membrane fusion through conformational change at low pH.

Hung-Wei Chang,Bo-Gang Su,Huei-Yin Cheng,Yi-Ting Liao,Der-Lii M Tzou,Shu-Yun Tung,Hao-Ching Wang,Kathleen Joyce D Carillo,Cheng-Han Yang,Wen Chang,Yu-Chun Luo

doi:10.1371/journal.ppat.1007826

Abstract

Vaccinia mature virus requires A26 envelope protein to mediate acid-dependent endocytosis into HeLa cells in which we hypothesized that A26 protein functions as an acid-sensitive membrane fusion suppressor. Here, we provide evidence showing that N-terminal domain (aa1-75) of A26 protein is an acid-sensitive region that regulates membrane fusion. Crystal structure of A26 protein revealed that His48 and His53 are in close contact with Lys47, Arg57, His314 and Arg312, suggesting that at low pH these His-cation pairs could initiate conformational changes through protonation of His48 and His53 and subsequent electrostatic repulsion. All the A26 mutant mature viruses that interrupted His-cation pair interactions of His48 and His 53 indeed have lost virion infectivity. Isolation of revertant viruses revealed that second site mutations caused frame shifts and premature termination of A26 protein such that reverent viruses regained cell entry through plasma membrane fusion. Together, we conclude that viral A26 protein functions as an acid-sensitive fusion suppressor during vaccinia mature virus endocytosis.

Highlights

Virus entry represents the initial stage of infection and is a target for developing new antiviral therapeutics
It is well established that vaccinia mature virus uses endocytosis to enter cells, it remains unclear how it triggers membrane fusion in the acidic environment of endosomes
We hypothesized that A26 protein in vaccinia mature virus functions as an acid-sensitive membrane fusion suppressor, which suggests a novel

Summary

Introduction

Virus entry represents the initial stage of infection and is a target for developing new antiviral therapeutics. An orthopoxvirus, is a model system for investigating poxvirus entry into host cells, producing mature (MV) and extracellular virus (EV) [2,3,4]. Vaccinia MV attaches to cell surface glycosaminoglycans and extracellular matrix laminin [5,6,7,8,9,10]. It clusters at lipid rafts, triggering the integrin β1-CD98-PI3K signaling cascade [11, 12] to induce actin-dependent endocytosis that may [13] or may not involve apoptotic mimicry [14,15,16]. Vaccinia MV is trafficked in vesicles inside the cells, with subsequent endosomal acidification triggering viral membrane fusion with the vesicular membrane to release viral cores into the cytoplasm [17,18,19,20]

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Conclusion