Filovirus Entry: A Novelty in the Viral Fusion World

Hunt, Catherine L.; Lennemann, Nicholas J.; Maury, Wendy

Amino acids 33 through 69 and three additional short downstream regions interact with GP 2 , serving as the base of the RBD. Several linearly discontinuous regions from amino acids 70 to 190 sit above the base forming a series of beta sheets. Residues both in beta sheets and adjacent loops have been implicated in cell binding, leading to the conclusion that the receptor binding site (RBS) is located in this region of the RBD [12, 13] . GP 1 residues 227 to 313 encode for a "glycan cap" that is extensively N-linked glycosylated and sits distal to the RBD from the surface of the virion. The glycan cap may protect the RBS from antibodies [3] . This glycan cap also interacts with two regions of GP 2 , including the internal fusion loop of GP 2 that is critical for GP 2 -mediated membrane fusion [7] . The glycan cap/GP 2 interaction restricts the availability of the fusion peptide, preventing pre-mature fusion events. Finally, filovirus GP 1 contains a mucin-like domain at the carboxy terminus from amino acids 310 to 511. This region is heavily glycosylated with both N-and O-linked glycans [7] . While the EBOV mucin domain is not required for virus entry [14, 15] , several roles for this domain have been suggested. Like the glycan cap, it may shield GP RBS residues from immune recognition on free virus [7] . In addition, the mucin domain causes cell rounding, masking of a number of cell surface markers and cytotoxicity that is not observed upon expression of mucin domain-deleted EBOV GP [15, 16] . The shielding effect of the bulky mucin domain of the RBD of GP 1 is thought to also obstruct RBS interactions with adherence factors and receptors since removal of this domain enhances EBOV titers. Similar attempts to delete the MARV mucin domain have proved unsuccessful [11] . Two of the three trimers are shown as space filling structures with GP 1 in lighter grey and GP 2 as dark grey/black. The third GP 1,2 heterodimer of the trimer is depicted as a ribbon structure with GP 1 shown in teal and the GP 2 subunit shown in tan. (C,D) Ribbon diagrams of a single heterodimer of GP 1,2 . Domains in GP 1 are highlighted in C, whereas domains in GP 2 are highlighted in D. In panel C, the base domain of GP 1 that interacts with GP 2 is shown in royal blue, the head domain is shown in teal with the beta-strands and adjacent loop region containing the RBS highlighted in red and the glycan cap is shown in gold. GP 2 is shown in grey. In panel D, the internal fusion loop (IFL) that is flanked by beta-strands is shown in dark brown and heptad repeat region 1 is shown in tan. The interaction of the IFL with GP 1 residues from an adjacent subunit is evident in panel A. All EBOV GP graphics (PDB accession number 3CSY) were produced with PyMol.