Helix-loop-helix motif in GnRH associated peptide is critical for negative regulation of prolactin secretion

The GnRH associated prolactin inhibiting factor (GAP) reveals the signature sequence associated with the helix-loop-helix structural motif. A number of different peptide fragments of GAP were designed, synthesized and analysed by circular dichroism and by an in vivo assay for prolactin secretion inhibiting activity. Peptides corresponding to the two individual alpha-helices and a 44-residue peptide comprising the entire helix-loop-helix motif show significant helical propensity in circular dichroism spectra. However, a peptide corresponding to the loop sequence shows no helical propensity. Albeit, the peptide corresponding to helix-loop-helix motif was found to inhibit prolactin secretion and augment circulating levels of gonadotropins in the in vivo assay; other shorter peptides did not show such activity. The activity profile of the 44-residue peptide was biphasic and very similar to that of the recombinant GAP. Thus, the prolactin inhibiting activity of this factor is defined by its helix-loop-helix motif as in the case of the transcription factors of developmental genes. The structural features of a homology-based model of GAP in complex with E47, a ubiquitous HLH-type developmental gene regulator, are consistent with the structural requirements of the negative regulation of transcription by helix-loop-helix proteins.