Influenza Virus M2 Protein Mediates ESCRT-Independent Membrane Scission

Jeremy S. Rossman, Xianghong Jing, George P. Leser, and Robert A. Lamb

2010. Cell. 142, 902-913

Speaker: Chun-Yung Lin (林峻暘)                               Time: 15:00~16:00, Dec. 8, 2010

Commentator: Dr. Jyh-Wei Shin (辛致煒博士)           Place: Room 601

Abstract:

Alterations in membrane curvature and scission at the neck of the budding virion are two main processes, which are critical for the budding of enveloped viruses. To achieve these, many viruses hijack the eukaryotic ESCRT (Endosomal Sorting Complex Required for Transport) pathway to mediate membrane scission and virion release (1). Previous studies indicate that influenza viruses may bud dependent on viral matrix (M2) protein, but independent on ESCRT proteins (1, 2). However, the mechanism of how M2 regulates this process is still unknown. First, the authors analyzed the amino acid sequence of M2 across all influenza subtypes and found a conserved M2 amphipathic helix (M2AH). It is suggested that the amphipathic helix has an important role for influenza virus. Using the large unilamellar vesicles (LUVs) and giant unilamellar vesicles (GUVs) model, they demonstrated that addition of wild type (wt) M2 to LUVs induced a negative membrane curvature in the presence of cholesterol. Low cholesterol GUVs containing wt M2 protein exhibited a rapid budding of the membrane and the formation of many intra-luminal vesicles (ILVs). No significant change in LUV/GUV morphology was observed when the vesicles were reconstituted with a mutant M2(AH-Mut) protein, suggesting that the M2AH is essential for M2-mediated membrane budding. Additionally, M2 expression led to the budding and release of M2-containing vesicles, but expression of M2(AH-Mut) led to a significant reduction in M2 vesicles. It is also confirmed that M2 caused membrane budding in vivo. Furthermore, examining the localization of M2 in virus-infected cells, the authors observed that M2 localized to the base of budding filamentous and sphericalvirions, and a specific localization at the neck of budding viruses. M2(AH-Mut) virus-infected cells, however, exhibited the ‘‘beads on a string’’ morphology. Taken together, M2 amphipathic helix is necessary and sufficient to mediate membrane scission, and suggest that M2 is responsible for mediating the budding of influenza virions.

References:

1.      Chen, B.J., et al. (2008). The influenza virus M2 protein cytoplasmic tail interacts with the M1 protein and influences virus assembly at the site of virus budding. J. Virol. 82, 10059–10070.

2.      Iwatsuki-Horimoto, et al. (2006). The cytoplasmic tail of the influenza A virus M2 protein plays a role in viral assembly. J. Virol. 80, 5233–5240.