Date Published: June 20, 2019
Publisher: Public Library of Science
Author(s): Hung-Wei Chang, Cheng-Han Yang, Yu-Chun Luo, Bo-Gang Su, Huei-Yin Cheng, Shu-Yun Tung, Kathleen Joyce D. Carillo, Yi-Ting Liao, Der-Lii M. Tzou, Hao-Ching Wang, Wen Chang, Richard C. Condit.
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.
Virus entry represents the initial stage of infection and is a target for developing new antiviral therapeutics. Poxvirus is a family of enveloped DNA viruses with genomes of ~200 kilobases . Vaccinia virus, an orthopoxvirus, is a model system for investigating poxvirus entry into host cells, producing mature (MV) and extracellular virus (EV) [2–4].
Poxviruses are very large and are known to contain multiple proteins of overlapping or redundant functions. Vaccinia virus contains four envelope proteins for cell attachment [5–8, 10], whereas viral membrane fusion requires a separate fusion protein complex of 11 components that specifically performs membrane fusion (reviewed in [3, 4]). Therefore, vaccinia virus has evolved two separate sets of envelope proteins specialized for cell attachment and membrane fusion, respectively, during cell entry.