The barbel chub (
Squaliobarbus curriculus) exhibits remarkable resistance to grass carp reovirus (GCRV), a devastating pathogen in aquaculture. To reveal the molecular basis of this resistance, we investigated complement factor D (DF)—a rate-limiting serine protease governing alternative complement pathway activation. Molecular cloning revealed that the barbel chub
DF (
ScDF) gene encodes a 1251-bp cDNA sequence translating into a 250-amino acid protein. Crucially, bioinformatic characterization identified a unique N-glycosylation site at Asn
139 in
ScDF, representing a structural divergence absent in grass carp (
Ctenopharyngodon idella) DF (
CiDF). While retaining a conserved Tryp_SPc domain harboring the catalytic triad (His
61, Asp
109, and Ser
204) and substrate-binding residues (Asp
198, Ser
219, and Gly
221), sequence and phylogenetic analyses confirmed
ScDF’s evolutionary conservation, displaying 94.4% amino acid identity with
CiDF and clustering within the Cyprinidae. Expression profiling revealed constitutive
ScDF dominance in the liver, and secondary prominence was observed in the heart. Upon GCRV challenge in
S. curriculus kidney (SCK) cells,
ScDF transcription surged to a 438-fold increase versus uninfected controls at 6 h post-infection (hpi;
p < 0.001)—significantly preceding the 168-hpi response peak documented for
CiDF in grass carp. Functional validation showed that
ScDF overexpression suppressed key viral capsid genes (
VP2,
VP5, and
VP7) and upregulated the interferon regulator
IRF9. Moreover, recombinant
ScDF protein incubation induced interferon pathway genes and complement
C3 expression. Collectively,
ScDF’s rapid early induction (peaking at 6 hpi) and multi-pathway coordination may contribute to barbel chub’s GCRV resistance. These findings may provide molecular insights into the barbel chub’s high GCRV resistance compared to grass carp and novel perspectives for anti-GCRV breeding strategies in fish.
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