TISSUE-DEPENDENT AND DEVELOPMENTALLY-REGULATED CYTOSOLIC THYROID-HORMONE-BINDING PROTEINS (CTBPS) IN XENOPUS

Xenopus cytosolic thyroid-hormone-binding proteins (CTBPs) were examin ed by a photoaffinity labeling and [I-125]T-3-binding assay. An affini ty-labeled protein of 59 kDa, which was responsible for the major T-3- binding activity in adult tissues, was predominant in liver. The 59-kD a CTBP first appeared in significant amounts at the metamorphic climax stage in liver cytosol and continued to be expressed after metamorpho sis. Another affinity-labeled CTBP of 38 kDa appeared at the metamorph ic climax stage in cytosol from head region, but disappeared after thi s stage. T-3-binding assay using whole cytosol showed that a distinct CTBP, although not photoaffinity-labeled, was present in cytosol from hindlimb bud and gradually disappeared as the hindlimb grew. The cytos ol from liver, head region, and hindlimb bud contained high affinity b inding sites for T-3 with K-d values ranging from 10(-9) to 10(-8) M. These results suggest that there are at least three distinct CTBPs in Xenopus cytosol, which are expressed in a tissue-dependent and develop mentally regulated manner. (C) 1997 Elsevier Science Inc.