Fiber1, but not fiber2, is the essential fiber gene for fowl adenovirus 4 (FAdV-4)

Fibre is the viral protein that mediates the attachment and infection of adenovirus to the host cell. Fowl adenovirus 4 (FAdV-4) possesses two different fibre trimers on each penton capsomere, and roles of the separate fibres remain elusive. Here, we attempted to investigate the function of FAdV-4 fibres by using reverse genetics approaches. Adenoviral plasmids carrying fiber1 or fiber2 mutant genes were constructed and used to transfect chicken LMH cells. Fiber1-mutated recombinant virus could not be rescued. Such defective phenotype was complemented when a fiber1-bearing helper plasmid was included for co-transfection. The infection of fiber-intact FAdV-4 (FAdV4-GFP) to LMH cells could be blocked with purified fiber1 knob protein in a dose-dependent manner, while purifed fiber2 knob had no such function. On the contrary, fiber2-mutated FAdV-4, FAdV4XF2-GFP, was successfully rescued. The results of one-step growth curves showed that proliferative capacity of FAdV4XF2-GFP was 10 times lower than that of the control FAdV4-GFP. FAdV4XF2-GFP also caused fewer deaths of infected chicken embryos than FAdV4-GFP did, which resulted from poorer virus replication in vivo. These data illustrated that fiber1 mediated virus adsorption and was essential for FAdV-4, while fiber2 was dispensable although it significantly contributed to the virulence.