Engrailed homeoprotein acts as a signaling molecule in the developing fly.

Homeodomain transcription factors classically exert their morphogenetic activities through the cell-autonomous regulation of developmental programs. In vertebrates, several homeoproteins have also been shown to have direct non-cell-autonomous activities in the developing nervous system. We present the first in vivo evidence for homeoprotein signaling in Drosophila. Focusing on wing development as a model, we first demonstrate that the homeoprotein Engrailed (En) is secreted. Using single-chain anti-En antibodies expressed under the control of a variety of promoters, we delineate the wing territories in which secreted En acts. We show that En is a short-range signaling molecule that participates in anterior crossvein development, interacting with the Dpp signaling pathway. This report thus suggests that direct signaling with homeoproteins is an evolutionarily conserved phenomenon that is not restricted to neural tissues and involves interactions with bona fide signal transduction pathways.