Partial Characterization of a Cerebral Thyroid Hormone-Responsive Protein

The thyroid hormone-responsive protein (THRP) is expressed in rat cerebral tissue and has 83% overall sequence homology with c-Abl interactor protein, Abi-2, which is a substrate for the tyrosine kinase activity of c-Abl. Within the core region of the two proteins, the sequence similarity approaches 99%. To determine whether THRP is a rat homologue of Abi-2 or is a distinct protein with unique properties, the tissue distribution of THRP and Abi-2 mRNAʹs was examined using a sensitive ribonuclease protection assay and probes specific for THRP and Abi-2, respectively. The THRP mRNA content of cerebral tissue (1340.0 ± 126.5 arbitrary units) was 2.3-fold higher than Abi-2 mRNA (581.3 ± 73.7), while the ratio of hepatic content of THRP mRNA (209.0 ± 49.1) to hepatic Abi-2 mRNA (2923.0 ± 378.7) was only 0.07 (P < 0.004). Very low levels of Abi-2 mRNA, but not THRP mRNA, were also found in the heart and small intestine. Experiments with PC12 cells transfected with the full-length THRP cDNA and grown in the presence or absence of a tyrosine kinase inhibitor, along with experiments where PC12 cells were cotransfected with the THRP cDNA with or without the wild-type or mutant (tyrosine kinase deficient) c-Abl cDNA, showed that THRP is tyrosine phosphorylated; however, it is not a substrate for c-Abl. These studies demonstrate that THRP and Abi-2 have distinct tissue distribution and distinct biological properties.