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 sign ature sequence associated with the helix-loop-helix structural motif. A number of different peptide fragments of GAP were designed, synthesi zed and analysed by circular dichroism and by an in vivo assay for pro lactin secretion inhibiting activity. Peptides corresponding to the tw o individual a-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 seque nce shows no helical propensity. Albeit, the peptide corresponding to helix-loop-helix motif was found to inhibit prolactin secretion and au gment circulating levels of gonadotropins in the in vivo assay; other shorter peptides did not show such activity. The activity profile of t he 44-residue peptide was biphasic and very similar to that of the rec ombinant GAP. Thus, the prolactin inhibiting activity of this factor i s defined by its helix-loop-helix motif as in the case of the transcri ption factors of developmental genes. The structural features of a hom ology-based model of GAP in complex with E47, a ubiquitous HLH-type de velopmental gene regulator, are consistent with the structural require ments of the negative regulation of transcription by helix-loop-helix proteins. (C) 1997 Academic Press Limited.