[hirenj]

Catflap is a surprisingly apt analogy. In this case it is an endocytosis receptor, that selects cargo for uptake. Differs from virus to virus of course, but I could see changes in the sugars on these proteins altering behaviour. Mucus/mucins are basically proteins very heavily modified with sugars, so you have this common system that is adapted to do different things.

[card_zero]

(endocytosis, into-cell-doing.)