TRANSTHYRETIN GENE-EXPRESSION IN CHOROID-PLEXUS 1ST EVOLVED IN REPTILES

The presence of transthyretin in mammals and birds, but not amphibia, suggested that transthyretin expression first appeared in stem reptile s. Therefore, transthyretin synthesis was studied in a lizard. Transth yretin synthesis in choroid plexus pieces from Tiliqua rugosa was demo nstrated by incorporation of radioactive amino acids. Oligonucleotides corresponding to conserved regions of transthyretin were used as prim ers in polymerase chain reaction with lizard choroid plexus cDNA. Ampl ified DNA was used to screen a lizard choroid plexus cDNA library. A f ull-length transthyretin cDNA clone was isolated and sequenced. A thre e-dimensional model of lizard transthyretin was obtained by homology m odeling. The central channel of transthyretin, containing the thyroxin e-binding site, was found to be completely conserved between reptiles and mammals. Transthyretin expression was not detected in lizard liver . These data suggest that transthyretin first evolved in the choroid p lexus of the brain. Due to a change in tissue distribution of gene exp ression, occurring much later during evolution, transthyretin also bec ame a plasma protein, synthesized in the liver.