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Expression of the Extracellular Domain of the Thyrotropin Receptor in the Baculovirus System Using a Promoter Active Earlier than the Polyhedrin Promoter

Conventional baculovirus vectors that utilize the very late polyhedrin promoter have not proved successful for expressing a thyrotropin (TSH) receptor capable of ligand and Graves' disease autoantibody binding comparable to the receptor produced in mammalian cells. Because of the clinical importance of high level expression of this protein, we reassessed the baculovirus system using a new transfer vector (pAcMP3) containing the late basic protein promoter, which functions earlier than the classical polyhedrin promoter. Maximal synthesis of the [S]methionine-labeled TSH receptor extracellular domain, affinity-purified using a 6-histidine tag, occurred earlier (1 day after insect cell infection) than with a vector (pVL1393) containing the polyhedrin promoter. The pAcMP3-derived TSH receptor extracellular domain was larger (68 kDa) than the pVL1393-derived protein (63 kDa). Only the 68-kDa product was secreted, albeit in trace amounts detectable only by precursor labeling. Enzymatic deglycosylation reduced both 68- and 63-kDa cellular proteins to 54 kDa, indicating that the pAcMP3 vector generated a protein with greater carbohydrate content. However, despite its greater degree of glycosylation, most of the 68-kDa protein remained within the cell, almost entirely in the particulate fraction. Remarkably, the trace amounts of 68-kDa receptor protein affinity-purified from the soluble cytosolic fraction of infected insect cells completely neutralized TSH receptor autoantibodies in patients' sera and partly inhibited TSH binding. In conclusion, a baculovirus vector with a promoter active earlier than the conventional polyhedrin promoter generates a more glycosylated and functional TSH receptor extracellular domain protein, albeit at low levels. These data carry important implications for the expression by baculovirus vectors of functional, highly glycosylated proteins.