RSV fusion: time for a new model

Abstract

In this review we propose a partially hypothetical model of respiratory syncytial virus (RSV) binding and entry to the cell that includes the recently discovered RSV receptor nucleolin, in an attempt to stimulate further inquiry in this research area. RSV binding and entry is likely to be a two-step process, the first involving the attachment of the virus to the cell membrane, which may be enhanced by electrostatic interactions with cellular glycoproteins/heparin and the viral G protein, and the second involving fusion to the cell membrane mediated by the viral F protein and a specific cellular fusion receptor. With our recent discovery of nucleolin as a functional fusion receptor for RSV, comes the possibility of a number of new approaches to the development of novel strategies for RSV prophylaxis and therapy, as well as raising some new questions concerning the pathobiology of RSV infection and tropism.

Figure 1

(A) Model of RSV F-protein binding nucleolin. On the left, the F-protein is shown in its trimeric pre-fusion conformation. The red circles are putative nucleolin binding sites. Nucleolin is shown in orange as part of a protein complex that includes proteins anchored to the membrane by either a transmembrane domain or a GPI anchor. Only one nucleolin molecule is shown binding the F-protein trimer for clarity but in this model as many as three could bind at once. On the right the F-protein is shown in the “extended” conformation with fusion peptides (yellow) inserted into the cell membrane. After this step virus-cell membrane fusion would proceed without nucleolin. (B) Diagram of virus binding to the cell surface. Indicated in light yellow are lipid-rich domains/rafts. The virus is shown in a dark magenta covered with F-protein binding to nucleolin that is preferentially located in lipid-rich rafts or caveolae. A caveola is shown covered with caveolin (dark red). This in turn can enter the cell to form a caveozome and join the endosomal or lysosomal pathway. Our proposed model leaves open the possibilities that viral fusion may occur at the cell surface or in a caveozome/endosome/lysosome. Another possibility (not shown) is that virus enters via clathirin-coated pits (see text).

Figure 2

Human airway epithelial cells from bronchial brushings stained with a rabbit polyclonal antibody against human nucleolin (H-250, Santa Cruz Biotechnology Inc., Santa Cruz , CA). Note the positive nucleolin immunostaining (dark brown) at the apical surface of airway epithelial cells (arrows).