Secretory expression of thyroid hormone receptor using transgenic silkworms and its DNA binding activity.

Silkworms are economically important insects that have the ability to produce large amounts of silk. They have mass breeding methods and silk glands, which are specialized tissues that secrete silk fibroin and sericin. Thus, the production of recombinant proteins in a transgenic silkworm system is a promising approach. We developed a silkworm, Bombyx mori, as a host expression insect for recombinant proteins and successfully produced different proteins including antibodies, glycoproteins, and membrane receptors. The thyroid hormone receptor (TR) is a regulatory factor for many physiological phenomena. It is a lipophilic protein that has DNA-binding and ligand-binding domains. Based on our previous experiences, it was inferred that the recombinant TR easily formed aggregates and precipitates which is potentially due to an unstructured hinge domain. We applied the silkworm expression system to produce mice TR beta 1 that was fused with glutathione 5-transferase. Using 160 larvae, the yield of the recombinant GST-TR beta was approximately 4 mg, and the purified GST-TR beta completely retained its physiological activity. Our results indicated that the recombinant TRP was secreted extracellularly using the silk fibroin signal peptide sequence. Moreover, we found that the expression system of silkworms was applicable to nuclear proteins.