A novel fibrinogen-related protein with two coiled-coil regions from Chlamys farreri functions as immune recognition molecule

• A novel FREP with two coiled-coil regions was identified from Zhikong scallop. • Cf FREP protein was highly expressed in immune tissues. • Cf FREP expression was significantly up-regulated by LPS and PGN. • r Cf FREP could bind to PAMPs of LPS, Poly I:C, PGN, and GLU. Fibrinogen-related proteins (FREPs) have been reported to exhibit extensive expansion with remarkable structural diversity in invertebrate genomes, particularly in mollusks, which execute multifaceted functions ranging from non-self recognition to clot formation. In the present study, a FREP was identified in Zhikong scallop, Chlamys farreri (named as CfFREP), which contained a signal peptide, a conserved fibrinogen-related domain (FBG), and two additional coiled-coil regions. Cf FREP clustered closely with the FREP from Argopecten irradians in the phylogenetic tree. The endogenous Cf FREP protein was predominantly localized in immune-related tissues, including the hepatopancreas, gill, kidney, and haemocytes. The mRNA expression of Cf FREP in haemocytes was significantly upregulated at 12 h post stimulation with lipopolysaccharides (LPS) and peptidoglycan (PGN), exhibiting a 10.8-fold ( p < 0.05) and 6.7-fold ( p < 0.01) increase, respectively, compared to the PBS control group. However, no significant change was observed in response to β-glucan (GLU) ( p > 0.05). The recombinant protein (r Cf FREP) displayed varied binding activities towards four different pathogen-associated molecular patterns (PAMPs) in a dose-dependent manner, with much higher affinity towards LPS and Poly I:C compared to PGN and β-glucan. These findings collectively suggested that Cf FREP functioned as a canonical immune recognition molecule in scallop, which offers fresh perspectives on the functions of mollusk FREPs.