Viral Size Modulates Sendai Virus Binding to Cholesterol-Stabilized Receptor Nanoclusters

Binding to the host membrane is the initial infection step for animal viruses. Sendai virus (SeV), the model respirovirus studied here, utilizes sialic-acid-conjugated glycoproteins and glycolipids as receptors for binding. In a previous report studying single virus binding to supported lipid bilayers (SLBs), we found a puzzling mechanistic difference between the binding of SeV and influenza A virus (strain X31, IAV^X31). Both viruses use similar receptors and exhibit similar cooperative binding behavior, but whereas IAV^X31 binding was altered by SLB cholesterol concentration, which can stabilize receptor nanoclusters, SeV was not. Here, we propose that differences in viral size distributions can explain this discrepancy; viral size could alter the number of virus-receptor interactions in the contact area and, therefore, the sensitivity to receptor nanoclusters. To test this, we compared the dependence of SeV binding on SLB cholesterol concentration between size-filtered and unfiltered SeV. At high receptor density, the unfiltered virus showed little dependence, but the size-filtered virus exhibited a linear cholesterol dependence, similar to IAV^X31. However, at low receptor densities, the unfiltered virus did exhibit a cholesterol dependence, indicating that receptor nanoclusters enhance viral binding only when the number of potential virus-receptor interactions is small enough. We also studied the influence of viral size and receptor nanoclusters on viral mobility following binding. Whereas differences in viral size greatly influenced mobility, the effect of receptor nanoclusters on mobility was small. Together, our results highlight the mechanistic salience of both the distribution of viral sizes and the lateral distribution of receptors in a viral infection.