Many progestins have been developed for use in contraception, menopausal hormone therapy, and treatment of gynecological diseases. They are derived from either progesterone or testosterone, and they act by binding to the progesterone receptor (PR), a hormone-inducible transcription factor belonging to the nuclear receptor superfamily. Unlike mineralocorticoid, glucocorticoid, and androgen receptors, the steroid-receptor contacts that trigger the switch of the ligand-binding domain from an inactive to an active conformation have not yet been identified for the PR. With this aim, we solved the crystal structure of the ligand-binding domain of the human PR complexed with levonorgestrel, a potent testosterone-derived progestin characterized by a 13-ethyl substituent. Via mutagenesis analysis and functional studies, we identified Met909 of the helix 12 as the key residue for PR activation by both testosterone-and progesterone-derived progestins with a 13-methyl or a 13-ethyl substituent. We also showed that Asn719 contributes to PR activation by testosterone-derived progestins only, and that Met759 and Met909 are responsible for the high potency of 19-norprogestins and of 13-ethyl progestins, respectively. Our findings provide a structural guideline for the rational synthesis of potent PR agonist and antagonist ligands that could have therapeutic uses in women's health.
The human RPSA [ribosomal protein SA; also known as LamR1(laminin receptor 1)] belongs to the
ribosome but is also a membrane receptor for laminin, growth factors, prion, pathogens and the
anticarcinogen EGCG (epigallocatechin-gallate). It contributes to the crossing of the
blood–brain barrier by neurotropic viruses and bacteria, and is a biomarker of metastasis.
RPSA includes an N-terminal domain, which is folded and homologous to the prokaryotic RPS2, and a
C-terminal extension, which is intrinsically disordered and conserved in vertebrates. We used
recombinant derivatives of RPSA and its N- and C-domains to quantify its interactions with ligands
by in-vitro immunochemical and spectrofluorimetric methods. Both N- and C-domains
bound laminin with KD (dissociation constants) of 300 nM. Heparin
bound only to the N-domain and competed for binding to laminin with the negatively charged C-domain,
which therefore mimicked heparin. EGCG bound only to the N-domain with a
KD of 100 nM. Domain 3 of the envelope protein from yellow fever
virus and serotypes-1 and -2 of dengue virus bound preferentially to the C-domain whereas that from
West Nile virus bound only to the N-domain. Our quantitative in-vitro approach
should help clarify the mechanisms of action of RPSA, and ultimately fight against cancer and
infectious agents.
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