Homeoprotein Intercellular Tansport: Mechanisms, Significance and Applications

Abstract

In the past few years we have demonstrated that homeoprotein transcription factors can be secreted and internalized by live cells. The third helix of the homeodomain (DNA-binding domain) is necessary and sufficient for internalization, whereas secretion requires a different domain with nuclear export sequence properties. Homeoprotein secretion requires nuclear export followed by an association with cholesterol-rich rafts, and homeoprotein internalization correlates with the formation of inverted micelles and is independent of classical endocytosis. In addition to the possible important physiological functions of this new intercellular signaling mechanism, the properties of homeodomain third helices have led to development of the Penetratin family of peptidic vectors. Among the many applications of Penetratins, two will be described. First, it will be shown that the internalization of homeodomains in a gene trap library of ES cells can lead to the identification of homeoprotein transcriptional targets. This procedure demonstrated that the BPAG1 locus, coding for proteins that bind intermediate filaments in the nervous system and in the skin, is a target of Engrailed and of several other homeoproteins. This locus has been implicated in two pathologies, thus adding to the list of homeoprotein targets of physiopathological interest. A second application has been to internalize into nerve cells several parts of the amyloid precursor protein cytoplasmic tail and to investigate the functional properties of these domains in vivo and in vitro.