Widespread expression of the mRNA encoding a novel vitamin D/thyroxine dual binding protein in the turtle Trachemys...

Vitamin D-binding protein (DBP) is the major transport protein for the vitamin D sterols in a variety of mammalian and avian species. The DBP found in the emydid family of turtles is unique in that it exhibits high-affinity binding of both vitamin D and thyroxine (D/TBP). Sequence analysis has revealed that the emydid D/TBP is homologous to mammalian DBP and shares no homology with mammalian thyroxine-binding globulins. Northern blot analysis was used to examine the spatial profile of D/TBP transcription in the turtle Trachemys scripta. In both adults and hatchlings, two transcripts--the expected 1.5-kb full-length transcript and a second 0.8-kb transcript--were present in nearly all of the tissues examined. In adults, highest expression of the 1.5-kb transcript was seen in the kidney, gonad, and spleen, with lower levels in the liver and lung and no transcripts in skeletal muscle. In hatchlings, the full-length transcript was detected in a variety of tissues at similar levels. Injection of hatchlings with thyroxine increased levels of circulating D/TBP and transcript levels. These data are in marked contrast to observations in mammals in which transcription of DBP is confined predominantly to the liver. Further, the increase in circulating D/TBP associated with increased thyroidal activity may result from a direct or indirect activation of D/TBP transcription by thyroxine.