I thought I was finally getting developmental biology, and then I read about engrailed. Engrailed is a transcription factor that often represses expression of its target genes. However, Layalle and colleagues show that engrailed can have another function - as a signaling molecule. The authors use an elegant approach with single chain antibodies with a signal peptide to specifically knock-down extracellular function in Drosophila wings. This is then integrated in the powerful UAS-Gal system to drive expression of these antibodies in different regions to understand which part of the engrailed expression domain is involved in sending the signal and where the signal is received. It was quite convincing and made me question or at least think about what a gene expression pattern, even of a transcription factor, means. Engrailed is not the only transcription factor with known signaling functions, and there could be more yet to discover.
Reference: Layalle S, Volovitch M, Mugat B, Bonneaud N, Parmentier ML, Prochiantz A, Joliot A, Maschat F. (2011) Engrailed homeoprotein acts as a signaling molecule in the developing fly. Development. 138:2315-23.