A novel complement C1q A chain from marbled flounder, Pseudopleuronectes yokohamae: genome characterization, expression and potential role in antibacterial immunity.

This study systematically analyzed the molecular characteristics, tissue expression, and function of the marbled flounder (Pseudopleuronectes yokohamae) C1qA protein (PyC1qA) in fish immunity. PyC1qA contains a signal peptide, collagen-like region, and globular head. Its amino acid sequence and globular structure are highly conserved among multiple species, especially at key sites for pathogen recognition. Phylogenetic analysis showed that PyC1qA has a close evolutionary relationship with other marine fish (such as marbled flounder and European flounder) but is different from freshwater fish. Tissue expression analysis showed that PyC1qA is highly expressed in the skin, and pathogen stimulation experiments showed that its expression changes dynamically in multiple immune-related tissues, indicating that it plays an important role in humoral immunity. Functional studies have shown that recombinant PyC1qA (rPyC1qA) can bind to a variety of fish pathogens and significantly enhance the hemolytic and bactericidal abilities of fish serum. Co-immunoprecipitation experiments verified the specific interaction between PyC1qA and IgM, further supporting its role in regulating humoral immunity through the classical complement pathway. In addition, in vivo experiments showed that rPyC1qA inhibited the proliferation of pathogens in immune-related tissues, demonstrating its potential anti-infection ability. This study revealed for the first time the molecular and functional characteristics of marbled flounder C1qA protein, expanded our understanding of the fish complement system in immune defense, and provided an important theoretical basis for the prevention and control of aquaculture diseases.