The crystal structure of an orthomyxovirus matrix protein reveals mechanisms for self-polymerization and membrane association

Abstract

Many enveloped viruses encode a matrix protein. In the influenza A virus, the matrix protein M1 polymerizes into a rigid protein layer underneath the viral envelope to help enforce the shape and structural integrity of intact viruses. The influenza virus M1 is also known to mediate virus budding as well as the nuclear export of the viral nucleocapsids and their subsequent packaging into nascent viral particles. Despite extensive studies on the influenza A virus M1 (FLUA-M1), only crystal structures of its N-terminal domain are available. Here we report the crystal structure of the full-length M1 from another orthomyxovirus that infects fish, the infectious salmon anemia virus (ISAV). The structure of ISAV-M1 assumes the shape of an elbow, with its N domain closely resembling that of the FLUA-M1. The C domain, which is connected to the N domain through a flexible linker, is made of four α-helices packed as a tight bundle. In the crystal, ISAV-M1 monomers form infinite 2D arrays with a network of interactions involving both the N and C domains. Results from liposome flotation assays indicated that ISAV-M1 binds membrane via electrostatic interactions that are primarily mediated by a positively charged surface loop from the N domain. Cryoelectron tomography reconstruction of intact ISA virions identified a matrix protein layer adjacent to the inner leaflet of the viral membrane. The physical dimensions of the virion-associated matrix layer are consistent with the 2D ISAV-M1 crystal lattice, suggesting that the crystal lattice is a valid model for studying M1-M1, M1-membrane, and M1-RNP interactions in the virion.ll members of the Orthomyxoviridae family, including the influenza viruses A-D, thogotovirus, and isavirus, encode a matrix protein called M1. In the influenza A virus, M1 is produced by a colinear transcript made from the gene segment 7 (1). As one of the most abundantly made viral proteins, the influenza A virus M1 plays multiple roles during the virus life cycle. Upon viral entry, the acidification of the viral interior in the endosome weakens the interaction between M1 and the viral ribonucleoprotein complexes (vRNPs), thus allowing M1-free vRNPs to be imported to the nucleus for viral RNA replication and transcription (2, 3). As infection proceeds, newly synthesized M1 enters the nucleus to mediate the nuclear export of nascent vRNPs. A "daisy-chain" complex is formed with the vRNP binding to the C-terminal domain of M1 (4) and the N-terminal domain of M1 interacting with the nuclear export protein (NEP), also called NS2. Through its nuclear export signal (NES), NEP is specifically recognized by the cellular exportin Crm1, which then facilitates the transport of vRNPs across the nuclear membrane to the cytoplasm in a RanGTP-dependent manner (5). It has been shown that M1 binding to vRNP in the nucleus is able to block mRNA transcription (4, 6). In the cytosol, M1 interacts with the cytoplasmic tails of the glycoproteins HA and NA. Such interactions promote M1 association with lipid raft membranes and t...