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ABSTRACT
Overexpression of the porcine LH receptor (pLHR) ectodomain has been achieved using the baculovirusinsect cell system but mostly in an aggregated form with no secretion. In order to carry this out, new baculoviruses were selected to produce the pLHR ectodomain in insect Sf9 cells and caterpillars. In pLHR-P10–297 and pLHR-mel-319 baculoviruses, pLHR cDNA was under the control of the P10 promoter and the polyhedrin gene promoter respectively.
The constructs contained either the porcine signal peptide (pLHR-P10–297) or the insect signal peptide of melittin (pLHR-mol-319). Infected cells produced 1 × 105−3 × 105 receptors/cell 3 days after infection. The recombinant LH receptor ectodomains produced were secreted in a biologically active form and bound the hormone with high affinity. Infected caterpillars produced a larger amount of active pLHR ectodomain than insect cells. The products were not secreted into the haemolymph however.
Promoter and/or signal peptide modifications therefore enabled pLHR recombinant ectodomain secretion in a biologically active form, using the baculovirus—lepidopteran cell system. Moreover, moderate levels of expression seem to allow the production of biologically active ectodomain.
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ABSTRACT
Porcine LH receptor ectodomain was overexpressed in insect cells and lepidopteran larvae using the recombinant baculovirus expression system. A low multiplicity of infection yielded the largest active production, of approximately 107 receptors/cell or 3 μg active receptor/mg total protein in infected cells. The truncated ectodomain solubilized with Triton X-100 bound its ligand with a high affinity which was comparable with that of the native membrane receptor. Increasing the multiplicity of infection resulted in an optimum protein production of 0·6 mg receptor/mg total protein in infected cells. This receptor was largely inactive, probably trapped within aggregation pools. Active receptor could be recovered by dilution of the samples. No secretion of recombinant receptor was ever observed whatever the conditions of infection. Expression of the recombinant receptor in insect larvae was also tested. This low-cost system failed both to increase the amount of active receptor and to induce secretion into the haemolymph. Two methods remain for producing sizeable amounts of active receptor with this baculovirus/insect cell system. One relies on immunoaffinity purification of the active protein and requires large-scale production, and the other is based on the purification of overexpressed inactive receptor followed by renaturation.
