FURIN and infection: L2 functions in both virion assembly and the infection process, but it is concealed inside the L1 pentamer [13] and its neutralization epitopes are thought to be transiently exposed via furin cleavage during the infection process; this is guided by the interaction between L1 and a heparan sulfate proteoglycan (HSPG), which triggers a surface conformational change of the viral capsid [18,19].