A novel C1q domain-containing protein from Tridacna crocea exhibits dual functionality in symbiont recognition and...

C1q domain-containing (C1qDC) proteins function as versatile pattern recognition receptors that mediate host-microbe interactions through their C-terminal C1q domains. In this study, a novel C1qDC protein named TcC1qDC was characterized from Tridacna crocea, featuring a 690 bp open reading frame encoding 229 amino acids. TcC1qDC exhibited constitutive but tissue-enriched expression, with the highest transcript levels in the outer mantle and hepatopancreas. Functional analyses revealed that recombinant TcC1qDC protein not only binds to symbiotic dinoflagellates but is also predicted to recognize multiple microbial carbohydrates, as demonstrated by molecular docking. Furthermore, this protein also displayed broad-spectrum binding activity against pathogen-associated molecular patterns (lipopolysaccharides, peptidoglycan, lipoteichoic acids, and mannan) and corresponding microorganisms, along with significant microbial agglutination capacity for Gram-negative bacteria, Gram-positive bacteria, and fungi. These findings collectively establish TcC1qDC as a dual-function receptor bridging symbiont recognition and immune defense in giant clams.