Sequence of a cDNA encoding turtle riboflavin-binding protein: a comparison with avian riboflavin-binding protein.

To search for the existence and distribution of a riboflavin-binding protein (RfBP), total RNA from estrogen-treated oviparous animals were screened by Northern hybridization using chicken RfBP cDNA as a probe. Besides avian livers and oviducts, RfBP mRNA was found in turtle liver, but not in the turtle oviduct. To elucidate the structure of the RfBP from a reptilian source, we constructed a cDNA library from estrogen-injected turtle liver, and a full-length turtle RfBP-encoding cDNA was cloned and sequenced. The open reading frame (ORF) encoded 242 amino acids (aa) including a signal peptide of 18 aa. There is an overall 71.3% aa identity between the deduced aa sequences of turtle and chicken. The aa sequence of turtle and chicken RfBP also show more than 30% similarity to a fragment of folate-binding protein (FBP). Six Trp and nine pairs of Cys residues are conserved between the two RfBPs with only one pair of Cys residues missing in FBP. The two Asn-linked glycosylation sites found in chicken RfBP are conserved in turtle RfBP, but only one of which is conserved in FBP. However, there is an additional potential N-glycosylation site in the turtle sequence and this may provide a better explanation for the greater molecular weight of the turtle protein than chicken RfBP. Turtle RfBP contains a region of nine Ser and five Glu residues which is present in mature chicken RfBP as eight phosphorylserine clusters forming a highly anionic region at the C terminus, but this region is not found in FBP.