EVIDENCE THAT A CALMODULIN-LIKE CALCIUM-BINDING DOMAIN OF THE FSH BETA-SUBUNIT IS INVOLVED IN FSH-INDUCED CALCIUM-UPTAKE BY SERTOLI CELLS

We have previously shown that a synthetic peptide amide corresponding to residues 1-15 of the human FSH beta-subunit (hFSH-beta-(1-15)) possesses structural characteristics and calcium-binding properties similar to the calcium-binding loops of calmodulin (CaM). The calcium-binding property of hFSH-beta-(1-15) correlated well with its ability to stimulate uptake of calcium (as Ca-45(2+)) by cultured rat Sertoli cells and proteoliposomes enriched with bovine calf testis FSH receptors. A sequence found in the calcium-binding loops of CaM and a number of other calcium-binding proteins can be represented by the motif +-+-+-+-+--+, where + represents a calcium-binding residue and - represents a non-binding residue. A sequence containing a similar motif appears in hFSH-beta-(1-15) between residues 4 and 15: +-++-+---+-+. Using a synthetic peptide strategy, we undertook to determine whether the first three residues of hFSH-beta-(1-15) were required to induce uptake of calcium by cultured rat Sertoli cells and FSH receptor-enriched proteoliposomes, and to assess whether rearrangement of the putative calcium-binding ligands (+) of hFSH-beta-(1-15) to correspond to their linear sequence in CaM would enhance the ability of hFSH-beta-(1-15) to induce calcium uptake in these two model systems. Our results indicate that (1) the amino terminal tripeptide of hFSH-beta-(1-15), NSC, is not required for its effects on calcium influx and (2), although the putative calcium-binding loop of hFSH-beta-(1-15) does not strictly adhere to the structural motif present in the calcium-binding loops of CaM, this does not adversely affect the potency of hFSH-beta-(1-15) in the systems studied. In addition to providing a structural basis for understanding the affinity of hFSH-beta-(1-15) for calcium, these studies suggest that the effects of FSH on calcium flux in Sertoli cells may involve a CaM-like region of its beta-subunit.