Role of Specific Transferrin Receptor Binding to Capsids
Understanding the interactions of the viral capsids with their host cell receptors
A key factor in understanding the host range evolution of the viruses is the cellular receptor. Canine parvovirus (CPV) and feline panleukopenia virus (FPV) capsids bind the transferrin receptor type-1 (TfR) of their host cells and tissues, and they use those receptors to infect cells. The binding is host specific, as FPV-like viruses bind to the feline TfR and to those from other hosts, but not the canine TfR. In contrast the CPV capsids also bind to the canine TfRs. Receptor binding affinity on host cells only partially correlates with the host ranges of the viruses, and an intermediate virus strain (CPV type-2) binds to higher levels on cells than did either the feline panleukopenia virus or a later strain of CPV. During the process of adaptation to dogs that later variant strains of CPV (called CPV-2a) gained the ability to both more efficiently use the canine TfR for infection, and also showed reduced binding to feline and canine cells compared to CPV type-2. Differences on the top and the side of the three fold spike of the capsid surface control specific TfR and other receptor binding, and also determine the infection properties of the viruses.
Mutagenesis of the Feline and Canine TfR Receptors
The structure of the TfR (from the human TfR) with sites marked that are clearly associated in the control of the binding of either FPV or CPV.
The importance of the TfR in conferring host range is highlighted by a number of targeted mutations. Substitution of Leu221 in the feline TfR with serine or aspartic acid stops the receptor binding to both FPV and CPV capsids, and when Leu221 is replaced with lysine the receptor still binds well to CPV but poorly to FPV. Analysis of recombinants between the feline and canine TfRs shows that sequences controlling virus-specific binding are within the apical domain (see the receptor structure page for more details). However, more than one sequence difference between those receptors is required to endow the feline TfR with the CPV-specific binding of the canine TfR. Single changes within the canine TfR which remove a single amino acid insertion or eliminate a unique glycosylation site give that receptor the ability to bind FPV. Binding of capsids to some mutant receptors prevents a productive infection, suggesting a specific role for the receptor in cell infection by the viruses. Investigations to identify additional sites on the receptor which have an effect on host range are ongoing.
TfR-capsid complex structures
To define structurally the sites on the virus and the TfR that were associated with binding, we used cryoEM analysis to define the structures of the feline TfR ectodomain in a mixture with CPV capsids. This showed that there were only a small number of TfRs associated with each capsid – perphaps as few as between 1 and 5 per capsid. In this analysis the position of one TfR was located, and that showed density attached to the capsid on the raised region near the threefold axis of symmetry of the capsid. We are following up on these studies by examing the structures at higher resolution and by further analysis of the specific residues that control receptor binding and cell infection.
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