Cytochrome b561 (cytb561)
proteins comprise a family of transmembrane
oxidoreductases that transfer single electrons across a membrane. Most eukaryotic species, including insects, possess multiple cytb561 homologs. To learn more about this
protein family in insects, we carried out a bioinformatics-based investigation of cytb561 family members from nine species representing eight insect orders. We performed a phylogenetic analysis to classify insect cytb561 ortholog groups. We then conducted sequence analyses and analyzed
protein models to predict structural elements that may impact the
biological functions and localization of these
proteins, with a focus on possible
ferric reductase activity. Our study revealed three ortholog groups, designated CG1275, Nemy, and CG8399, and a fourth group of less-conserved genes. We found that CG1275 and Nemy
proteins are similar to a human
ferric reductase, duodenal
cytochrome b561 (Dcytb), and have many conserved
amino acid residues that function in substrate binding in Dcytb. Notably, CG1275 and Nemy
proteins contain a conserved
histidine and other residues that play a role in ferric ion reduction by Dcytb. Nemy
proteins were distinguished by a novel
cysteine-rich cytoplasmic loop sequence. CG8399 orthologs are similar to a putative
ferric reductase in humans, stromal cell-derived receptor 2. Like other members of the CYBDOM class of cytb561
proteins, these
proteins contain reeler, DOMON, and cytb561 domains. Drosophila melanogaster CG8399 is the only insect cytb561 with known
ferric reductase activity. Our investigation of the DOMON domain in CG8399
proteins revealed a probable
heme-binding site and a possible site for ferric reduction. The fourth group includes a subgroup of
proteins with a conserved "KXXXXKXH" non-cytoplasmic loop motif that may be a substrate binding site and is present in a potential
ferric reductase, human
tumor suppressor
cytochrome b561. This study provides a foundation for future investigations of the
biological functions of cytb561 genes in insects.