A Putative Receptor for Ferritin in Mollusks: Characterization of the Insulin-like Growth Factor Type 1 Receptor.

The ferritin secreted by mammals has been well documented, with the protein capable of localizing to cell membranes and facilitating the delivery of iron to cells through endocytosis. However, the presence of ferritin in the circulatory fluid of mollusks and its functions remain largely unknown. In this study, we aimed to investigate the potential interacting proteins of ferritin in the ark clam (Fn) through the use of a pull-down assay. Our findings revealed the presence of an insulin-like growth factor type 1 receptor (IGF-1R) in ark clams, which was capable of binding to Fn and was named IGF-1R. IGF-1R was found to be composed of two leucine-rich repeat domains (L domain), a cysteine-rich domain, three fibronectin type III domains, a transmembrane domain, and a tyrosine kinase domain. The ectodomain of IGF-1R was observed to form a symmetrical antiparallel homodimer in the shape of the letter 'A', with the fibronectin type III domains serving as its 'legs'. The mRNA expression of IGF-1R gene was detected ubiquitously in various tissues of the ark clam, with the highest expression levels found in hemocytes, as determined by qRT-PCR. Using a confocal microscopic and yeast two-hybrid assays, the interaction between IGF-1R and Fn was further verified. The results showed that Fn co-localized with IGF-1R on the cell membrane, and their interaction was expected to occur on the FNIII domains of the IGF-1R. In conclusion, our findings highlight the identification of a putative receptor, IGF-1R, for Fn, demonstrating the versatility of IGF-1R in ark clams.