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Ptx1 regulates SF‐1 activity by an interaction that mimics the role of the ligand‐binding domain

Ptx1 (Pitx1) is a bicoid‐related homeobox transcription factor expressed from the onset of pituitary development. It was shown to cooperate with cell‐restricted factors, such as Pit1, NeuroD1/PanI and steroidogenic factor 1 (SF‐1), to establish a combinatorial code conferring lineage‐ and promoter‐specific gene transcription in the pituitary. Transcriptional synergism between Ptx1 and SF‐1 on two SF‐1 target genes, pituitary luteinizing hormone β and Müllerian‐inhibiting substance (MIS), requires SF‐1 binding to DNA and appears to result from direct physical interaction between these two proteins. The interaction between the C‐terminus of Ptx1 and the N‐terminal half of SF‐1 results in transcriptional enhancement that equals the activity of a constitutively active SF‐1 mutant and that may mimic the effect of a still unidentified SF‐1 ligand. Thus, the unmasking of SF‐1 activity by Ptx1 may represent a developmental mechanism to alleviate the need for SF‐1 ligand in transcription and, possibly, at critical times during organogenesis.