The structure of Manduca sexta transferrin-1 reveals a novel method of iron coordination

The secreted protein, transferrin-1 (Tsf1), has a role in maintaining the iron homeostasis of insects. This role is likely mediated through its ability to bind iron tightly and reversibly. The iron binding affinity of Tsf1 from Drosophila melanogaster (DmTsf1) and Manduca sexta (MsTsf1) is high (log K’ = 18 at pH 7.4), and iron release occurs as the pH decreases. These iron binding and release properties of Tsf1 are similar to the homologous proteins found in vertebrates, lactoferrin, serum transferrin and ovotransferrin; however, bioinformatic and spectroscopic properties of DmTsf1 and MsTsf1 compared to vertebrate transferrins suggest differences in how Tsf1 coordinates iron. The goal of this study was to further our understanding of how Tsf1 coordinates iron. To address this question, we performed structural analysis of the first crystal structure (2.05 Å resolution) of an insect transferrin (MsTsf1). MsTsf1 was purified from larval hemolymph in its iron bound and glycosylated form. The structure has an overall bilobal fold, similar to vertebrate transferrins, with a distinct amino- and carboxyl- lobe. The structure revealed a single iron bind site in the amino-lobe, with iron coordination occurring through the use of two tyrosines and two carbonate anions. These two tyrosines and the residues that coordinate the carbonate anions are conserved in Tsf1 orthologs. This method of iron coordination in MsTsf1 is novel amongst transferrin structures and answers our question of how iron coordination occurs in Tsf1. These findings provide a structural basis for understanding Tsf1 function in iron sequestration and transport.