Breaking in: human metapneumovirus fusion and entry

Abstract

Human metapneumovirus (HMPV) is a leading cause of respiratory infection that causes upper airway and severe lower respiratory tract infections. HMPV infection is initiated by viral surface glycoproteins that attach to cellular receptors and mediate virus membrane fusion with cellular membranes. Most paramyxoviruses use two viral glycoproteins to facilitate virus entry-an attachment protein and a fusion (F) protein. However, membrane fusion for the human paramyxoviruses in the Pneumovirus subfamily, HMPV and respiratory syncytial virus (hRSV), is unique in that the F protein drives fusion in the absence of a separate viral attachment protein. Thus, pneumovirus F proteins can perform the necessary functions for virus entry, i.e., attachment and fusion. In this review, we discuss recent advances in the understanding of how HMPV F mediates both attachment and fusion. We review the requirements for HMPV viral surface glycoproteins during entry and infection, and review the identification of cellular receptors for HMPV F. We also review our current understanding of how HMPV F mediates fusion, concentrating on structural regions of the protein that appear to be critical for membrane fusion activity. Finally, we illuminate key unanswered questions and suggest how further studies can elucidate how this clinically important paramyxovirus fusion protein may have evolved to initiate infection by a unique mechanism.

Figure 1

Schematic representation of a human metapneumovirus (HMPV) virion. The fusion (orange), attachment (red), and short hydrophobic (black) glycoproteins are depicted at the virion surface. The matrix protein (gray ovals) lines the inner leaflet of the virus membrane. Encapsidated within the viral envelope is the ribonucleoprotein (RNAP) complex consisting of the helical, genomic RNA wrapped by the nucleoprotein (N), the viral RNA-dependent, RNA polymerase (L), phosphoprotein (P), and matrix 2 protein (M2).

Figure 2

Schematic representation of the cleaved HMPV fusion protein. The mature, proteolytically cleaved HMPV F protein contains 522 amino acids (without signal sequence), with 82 residues in the F2 subunit and 440 residues in the F1 subunit, which includes a large extracellular domain, an ~23-amino acid transmembrane domain and a cytoplasmic tail of 25 residues. FP = fusion peptide; HRA = heptad repeat A; HRB = heptad repeat B; TM = transmembrane domain; CT = cytoplasmic tail. The approximate location of a conserved RGD motif (residues 329–331) is indicated as a magenta box. Arrows indicate the three N-linked glycosylation sites. The location of two disulfide bonds that connect the F1 and F2 protein subunits are shown. (Scale bar, amino acids.)

Figure 3

Structures of paramyxovirus F proteins. (A) The prefusion conformation of the PIV5 F trimer and the postfusion conformation of the hRSV F trimer. A schematic of the F protein color-coded to identify structural domains is shown below the structures. The DI domain is shown in orange, the DII domain is shown in red, and the DIII domain is shown in blue. HRA is shown in light blue, HRB is shown in cyan and the HRB linker domain is shown in gray. The fusion peptide is shown in green for prefusion PIV5 F, but was not solved in the hRSV F structure. The GCNt domain (shown in yellow) was added to the PIV5 F trimer for crystallization. (B) HMPV F in complex with a neutralizing antibody (DS7 Fab, shown as gray surface). DI, DII, and DIII coloring is as for the F trimers shown in (A). The conserved RGD motif is shown as magenta spheres. Residue 294 (Glu) is shown as green spheres. Conserved charged residues in DIII that impact fusion activity are indicated as spheres (yellow, acidic residues; brown, basic residues). (C) Side and top views of the prefusion PIV5 F trimer are shown with the homologous residues to HMPV F-RGD shown as magenta spheres. Each monomer is shown in blue, yellow, or green. (D) Side and top views of the postfusion hRSV F trimer are shown with the homologous residues to HMPV F-RGD shown as magenta spheres. Coloring is as for PIV5 F in (C). Structural coordinates were obtained from the Protein Data Bank [52] and figure constructed with PyMOL. PIV5 F, PDB ID: 2B9B [53]; hRSV F, PDB ID: 3RKI [54]; HMPV F complex, PDB ID: 4DAG [55].