We have recently shown that addition of follitropin (FSH) or a phorbol ester (phorbol 12-myristate 13-acetate (PMA)) to cells expressing the recombinant follitropin receptor (FSHR) results in both phosphorylation and uncoupling of the FSHR from adenylyl cyclase. In the light of findings reported with other G protein-coupled receptors we have proposed that phosphorylation of the FSHR mediates the uncoupling from adenylyl cyclase. The experiments described herein represent the first attempt to determine the location of the amino acid residues that become phosphorylated in FSHR and to test the hypothesis that phosphorylation is responsible for uncoupling of FSHR from adenylyl cyclase.

As a first step in identifying which residues may be phosphorylated in response to hFSH and PMA, we constructed a mutant of the FSHR cDNA in which the C-terminal cytoplasmic tail was truncated at residue 635 (FSHR-t635), thus removing all but one of the potential phosphorylation sites present in the C-terminal tail. Cells expressing FSHR-t635 bind hFSH with the appropriate affinity and respond with increases in cAMP and inositol phosphate accumulation. The maximal cAMP and inositol phosphate responses of cells expressing FSHR-t635 are higher than those of cells expressing the wild type FSHR, but the concentration of hFSH required to elicit these responses is similar in both cell lines.

Immunoprecipitation of FSHR-t635 shows that the truncated receptor is still effectively phosphorylated in response to hFSH or PMA. Phosphoamino acid analysis reveals that, like the wild-type FSHR, FSHR-t635 phosphorylation occurs on serine and threonine residues. Peptide mapping suggests that the phosphorylated residues in the FSHR and FSHR-t635 are located within the same areas of the intracellular regions of the receptors. In addition to stimulating phosphorylation of FSHR-t635, hFSH and PMA also effectively uncouple the truncated receptor from adenylyl cyclase. Taken together, these data show that hFSH and PMA can both phosphorylate and uncouple a FSH receptor species with a cytoplasmic tail truncated at residue 635.

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