Dj. Whitworth et al., Widespread expression of the mRNA encoding a novel vitamin D/thyroxine dual binding protein in the turtle Trachemys scripta, GEN C ENDOC, 118(2), 2000, pp. 354-358
Vitamin D-binding protein (DBP) is the major transport protein for the vita
min 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 b
inding of bath vitamin D and thyroxine (D/TBP). Sequence analysis has revea
led that the emydid D/TBP is homologous to mammalian DBP and shares no homo
logy with mammalian thyroxine-binding globulins. Northern blot analysis was
used to examine the spatial profile of D/TBP transcription in the turtle T
rachemys scripta. In both adults and hatchlings, two transcripts-the expect
ed 1.5-kb full-length transcript and a second 0.8-kb transcript-were presen
t in nearly all of the tissues examined. In adults, highest expression of t
he 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 hatc
hlings, 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 predom
inantly to the liver. Further, the increase in circulating D/TBP associated
with increased thyroidal activity may result from a direct or indirect act
ivation of D/TBP transcription by thyroxine. (C) Academic Press.