Intravirion cohesion of matrix protein M1 with ribonucleocapsid is a prerequisite of influenza virus infectivity

New structural insights into the multifunctional influenza A matrix protein 1

Influenza A virus matrix protein 1 (M1) is the most abundant protein within virions and functions at multiple steps of the virus life cycle, including nuclear RNA export, virus particle assembly, and virus disassembly. Two recent publications have presented the first structures of full‐length M1 and show that it assembles filaments in vitro via an interface between the N‐ and C‐terminal domains of adjacent monomers. These filaments were found to be similar to those that form the endoskeleton of assembled virions. The structures provide a molecular basis to understand the functions of M1 during the virus life cycle. Here, we compare and discuss the two structures, and explore their implications for the mechanisms by which the multifunctional M1 protein can mediate virus assembly, interact with viral ribonucleoproteins and act during infection of a new cell.

The cleavage of spike protein НА0→НА1/HA2 by trypsin permits activation of the M2 channel without its proteolytic cleavage in the influenza A virus

M2 plays numerous regulatory roles in influenza A virus infection confirming the old adage: "a little body often harbors a great sense". The comment here demonstrates that a small viral protein M2, having 14 kD m.w. and situating in the virion at a minor amount of only about 40 molecules per virus particle is resistant to trypsin at concentrations initiating the HA0 cleavage and virus infectivity activation. A mechanism involving a programmed disassembly by cascade-type transmembrane signaling of the HA-M2-M1-RNP cooperation during virus entry into the infected cell is proposed.

Hemagglutinin Cleavability, Acid Stability, and Temperature Dependence Optimize Influenza B Virus for Replication in Human Airways

Influenza A virus (IAV) and influenza B virus (IBV) cause yearly epidemics with significant morbidity and mortality. When zoonotic IAVs enter the human population, the viral hemagglutinin (HA) requires adaptation to achieve sustained virus transmission. In contrast, IBV has been circulating in humans, its only host, for a long period of time. Whether this entailed adaptation of IBV HA to the human airways is unknown. To address this question, we compared two seasonal IAVs (A/H1N1 and A/H3N2) and two IBVs (B/Victoria and B/Yamagata lineages) with regard to host-dependent activity of HA as the mediator of membrane fusion during viral entry. We first investigated proteolytic activation of HA by covering all type II transmembrane serine protease (TTSP) and kallikrein enzymes, many of which proved to be present in human respiratory epithelium. The IBV HA0 precursor is cleaved by a broader panel of TTSPs and activated with much higher efficiency than IAV HA0. Accordingly, knockdown of a single protease, TMPRSS2, abrogated spread of IAV but not IBV in human respiratory epithelial cells. Second, the HA fusion pH values proved similar for IBV and human-adapted IAVs (with one exception being the HA of 1918 IAV). Third, IBV HA exhibited higher expression at 33°C, a temperature required for membrane fusion by B/Victoria HA. This indicates pronounced adaptation of IBV HA to the mildly acidic pH and cooler temperature of human upper airways. These distinct and intrinsic features of IBV HA are compatible with extensive host adaptation during prolonged circulation of this respiratory virus in the human population.IMPORTANCE Influenza epidemics are caused by influenza A and influenza B viruses (IAV and IBV, respectively). IBV causes substantial disease; however, it is far less studied than IAV. While IAV originates from animal reservoirs, IBV circulates in humans only. Virus spread requires that the viral hemagglutinin (HA) is active and sufficiently stable in human airways. We resolve here how these mechanisms differ between IBV and IAV. Whereas human IAVs rely on one particular protease for HA activation, this is not the case for IBV. Superior activation of IBV by several proteases should enhance shedding of infectious particles. IBV HA exhibits acid stability and a preference for 33°C, indicating pronounced adaptation to the human upper airways, where the pH is mildly acidic and a cooler temperature exists. These adaptive features are rationalized by the long existence of IBV in humans and may have broader relevance for understanding the biology and evolution of respiratory viruses.

Matrix proteins of enveloped viruses: a case study of Influenza A virus M1 protein

Influenza A virus, a member of the Orthomyxoviridae family of enveloped viruses, is one of the human and animal top killers, and its structure and components are therefore extensively studied during the last decades. The most abundant component, M1 matrix protein, forms a matrix layer (scaffold) under the viral lipid envelope, and the functional roles as well as structural peculiarities of the M1 protein are still under heavy debate. Despite multiple attempts of crystallization, no high resolution structure is available for the full length M1 of Influenza A virus. The likely reason for the difficulties lies in the intrinsic disorder of the M1 C-terminal part preventing diffraction quality crystals to be grown. Alternative structural methods including synchrotron small-angle X-ray scattering (SAXS), atomic force microscopy, cryo-electron microscopy/tomography are therefore widely applied to understand the structure of M1, its self-association and interactions with the lipid membrane and the viral nucleocapsid. These methods reveal striking similarities in the behavior of M1 and matrix proteins of other enveloped RNA viruses, with the differences accompanied by the specific features of the viral lifecycles, thus suggesting common interaction principles and, possibly, common evolutional ancestors. The structural information on the Influenza A virus M1 protein obtained to the date strongly suggests that the intrinsic disorder in the C-terminal domain has important functional implications.

Palmitoylation Contributes to Membrane Curvature in Influenza A Virus Assembly and Hemagglutinin-Mediated Membrane Fusion

The highly conserved cytoplasmic tail of influenza virus glycoprotein hemagglutinin (HA) contains three cysteines, posttranslationally modified by covalently bound fatty acids. While viral HA acylation is crucial in virus replication, its physico-chemical role is unknown. We used virus-like particles (VLP) to study the effect of acylation on morphology, protein incorporation, lipid composition, and membrane fusion. Deacylation interrupted HA-M1 interactions since deacylated mutant HA failed to incorporate an M1 layer within spheroidal VLP, and filamentous particles incorporated increased numbers of neuraminidase (NA). While HA acylation did not influence VLP shape, lipid composition, or HA lateral spacing, acylation significantly affected envelope curvature. Compared to wild-type HA, deacylated HA is correlated with released particles with flat envelope curvature in the absence of the matrix (M1) protein layer. The spontaneous curvature of palmitate was calculated by molecular dynamic simulations and was found to be comparable to the curvature values derived from VLP size distributions. Cell-cell fusion assays show a strain-independent failure of fusion pore enlargement among H2 (A/Japan/305/57), H3 (A/Aichi/2/68), and H3 (A/Udorn/72) viruses. In contradistinction, acylation made no difference in the low-pH-dependent fusion of isolated VLPs to liposomes: fusion pores formed and expanded, as demonstrated by the presence of complete fusion products observed using cryo-electron tomography (cryo-ET). We propose that the primary mechanism of action of acylation is to control membrane curvature and to modify HA's interaction with M1 protein, while the stunting of fusion by deacylated HA acting in isolation may be balanced by other viral proteins which help lower the energetic barrier to pore expansion.IMPORTANCE Influenza A virus is an airborne pathogen causing seasonal epidemics and occasional pandemics. Hemagglutinin (HA), a glycoprotein abundant on the virion surface, is important in both influenza A virus assembly and entry. HA is modified by acylation whose removal abrogates viral replication. Here, we used cryo-electron tomography to obtain three-dimensional images to elucidate a role for HA acylation in VLP assembly. Our data indicate that HA acylation contributes to the capability of HA to bend membranes and to HA's interaction with the M1 scaffold protein during virus assembly. Furthermore, our data on VLP and, by hypothesis, virus suggests that HA acylation, while not critical to fusion pore formation, contributes to pore expansion in a target-dependent fashion.

Lateral Organization of Influenza Virus Proteins in the Budozone Region of the Plasma Membrane

Influenza virus assembles and buds at the plasma membrane of virus-infected cells. The viral proteins assemble at the same site on the plasma membrane for budding to occur. This involves a complex web of interactions among viral proteins. Some proteins, like hemagglutinin (HA), NA, and M2, are integral membrane proteins. M1 is peripherally membrane associated, whereas NP associates with viral RNA to form an RNP complex that associates with the cytoplasmic face of the plasma membrane. Furthermore, HA and NP have been shown to be concentrated in cholesterol-rich membrane raft domains, whereas M2, although containing a cholesterol binding motif, is not raft associated. Here we identify viral proteins in planar sheets of plasma membrane using immunogold staining. The distribution of these proteins was examined individually and pairwise by using the Ripley K function, a type of nearest-neighbor analysis. Individually, HA, NA, M1, M2, and NP were shown to self-associate in or on the plasma membrane. HA and M2 are strongly coclustered in the plasma membrane; however, in the case of NA and M2, clustering depends upon the expression system used. Despite both proteins being raft resident, HA and NA occupy distinct but adjacent membrane domains. M2 and M1 strongly cocluster, but the association of M1 with HA or NA is dependent upon the means of expression. The presence of HA and NP at the site of budding depends upon the coexpression of other viral proteins. Similarly, M2 and NP occupy separate compartments, but an association can be bridged by the coexpression of M1.IMPORTANCE The complement of influenza virus proteins necessary for the budding of progeny virions needs to accumulate at budozones. This is complicated by HA and NA residing in lipid raft-like domains, whereas M2, although an integral membrane protein, is not raft associated. Other necessary protein components such as M1 and NP are peripherally associated with the membrane. Our data define spatial relationships between viral proteins in the plasma membrane. Some proteins, such as HA and M2, inherently cocluster within the membrane, although M2 is found mostly at the periphery of regions of HA, consistent with the proposed role of M2 in scission at the end of budding. The association between some pairs of influenza virus proteins, such as M2 and NP, appears to be brokered by additional influenza virus proteins, in this case M1. HA and NA, while raft associated, reside in distinct domains, reflecting their distributions in the viral membrane.