Transthyretin interacts with metallothionein 2

Transthyretin (TTR) is a 55 kDa homotetrameric protein known for the transport of thyroxine and the indirect transportation of retinol. Within the central nervous system, TTR is primary synthesized and secreted into the cerebral spinal fluid by the choroid plexus (CP), whereas most TTR in the systemic circulation is produced and secreted by the liver. TTR is involved in two types of amyloid disease, the senile systemic amyloidosis and the familial amyloidotic polyneuropathy. TTR has also been implicated in the sequestration of amyloid beta peptide (Abeta), preventing its deposition. To explore other biological roles for TTR, we searched for protein-protein interactions using the yeast two-hybrid system with the full-length human TTR cDNA as bait. We found a novel interaction between TTR and metallothionein 2 (MT2) in human liver. This interaction was confirmed by competition binding assays, co-immunoprecipitation, cross-linking, and Western blotting experiments. Binding studies using MT1 showed a saturable specific interaction with TTR with a Kd of 244.8 +/- 44.1 nM. Western blotting experiments revealed a TTR-MT1/2 protein complex present in rat CP and kidney tissue extracts. Immunofluorescence experiments, in CP primary cell cultures and in CP paraffin sections, showed co-localization of TTR and MT1/2 in the cytoplasm of epithelial CP cells and localization of MT1/2 in the endoplasmic reticulum. Moreover, dot blot immunoassays of rat CSF provided the first evidence, to our knowledge, of circulating metallothionein in CSF. Taken together, we suggest that TTR-MT1/2 complexes may be functionally significant not only in healthy conditions but also in Abeta deposition in Alzheimer disease, thereby providing a novel potential therapeutic target.