Résumé : Calmodulin (CaM) contributes to estrogen receptor α (ER)-mediated transcription. In order to study the underlying mechanisms, we synthesized a peptide including the CaM binding site: ERα17p (P295-T311). This peptide inhibited ER-CaM association, unlike two analogs in which two amino acids required for CaM binding were substituted. Exposure of MCF-7 cells to ERα17p down regulated ER, stimulated ER-dependent transcription and enhanced the proliferation of ER-positive breast cancer cell lines. Interestingly, ERα17p analogs unable to bind to CaM induced similar responses, demonstrating that ERα17p-mediated effects are mainly relevant to mechanisms independent of ER-CaM dissociation. The P295-T311 motif is indeed a platform for multiple post-translational modifications not necessarily CaM-dependent. The additional finding that deletion of the P295-T311 sequence in ER produced a constitutive transcriptional activity revealed that this platform motif has autorepressive functions. With regard to cell function, association of CaM to ER would counteract this autorepression, leading thereby to enhanced ER-mediated transactivation. © 2007 Elsevier Ireland Ltd. All rights reserved.