EXONS-1-10 OF THE RAT LH RECEPTOR ENCODE A HIGH-AFFINITY HORMONE BINDING-SITE AND EXON-11 ENCODES G-PROTEIN MODULATION AND A POTENTIAL 2ND HORMONE BINDING-SITE

We have reported that the rat LH receptor is encoded by 11 exons of a single copy gene. Exons 1-10 encode the N-terminal half and exon 11 the C-terminal half. Since exon splice sites often mark structural transitions of multiexon molecules, we have attempted to define the function of the exons by generating mutant receptors with missing exons. As a first step, we have constructed two LH mutant receptors, one containing exons 1-10 (LH receptor(exon 1-10)) and the other containing exon 1 and exon 11 (LH receptor(exon 1 & 11)). These mutant receptors were functionally expressed in Cos 7A cells. The LH mutant receptor(exon 1-10), which lacks the membrane associated C-terminal half of the receptor, showed a high affinity for hCG. Surprisingly, the LH mutant receptor(exon 1 & 11) recognized hCG with a low affinity and stimulated G-proteins and cAMP production. The results demonstrate that exons 1-10 encode a high affinity hCG binding site and proves an important hypothesis that exon 11 encodes the site for receptor-modulation to activate G-proteins. Furthermore, the results raises an intriguing possibility of a second hormone binding site in the C-terminal half and multistep hormone binding.