Vitamin D binding protein: genomic structure, functional domains, and mRNA expression in tissues

The vitamin D binding protein (DBP), alternatively known as Gc-globulin, is a member of the albumin (ALB) and alpha-fetoprotein (AFP) gene family. The rat DBP gene is expressed at high levels in liver and at moderate levels in kidney, testis, abdominal fat, and yolk sac. Very low levels of DBP as well as ALB and AFP transcripts can be detected in all other tissues studied by the reverse transcriptase/polymerase chain reaction technique. During development, liver DBP gene transcripts are detectable at 14 days of gestation and levels rise gradually until adulthood in parallel with ALB. DBP present on the surface of U937 monocyte-derived cells is acquired from serum, suggesting cell surface binding sites for DBP. The rat DBP gene has been cloned and characterized. It spans 35 kb and contains 13 exons and 12 introns. The DBP gene contains two fewer exons than the ALB or AFP genes, accounting for the shortest size of its mRNA and protein product. Its 5'-flanking region contains a high degree of structural similarity to both ALB and AFP promoters.