spen encodes an RNP motif protein that interacts with Hox pathways to repress the development of head-like sclerites in the Drosophila trunk

Drosophila has eight Hox proteins, and they require factors acting in parallel to regulate different segmental morphologies. Here we find that the Drosophila gene split ends (spen), has a homeotic mutant phenotype, and appears to encode such a parallel factor. Our results indicate that spell plays two important segment identity roles. One is to promote sclerite development in the head region, in parallel with Hox genes; the other is to cooperate with Antennapedia and teashirt to suppress head-like sclerite development in the thorax. Our results also indicate that without spen and teashirt functions, Antennapedia loses its ability to specify thoracic identity in the epidermis, spelt transcripts encode extraordinarily large protein isoforms (approx, 5,500 amino acids), which are concentrated in embryonic nuclei. Both Spen protein isoforms and Spen-like proteins in other animals possess a clustered repeat of three RNP (or RRM) domains, as well as a conserved motif of 165 amino acids (SPOC domain) at their C-termini, Spen is the only known homeotic protein with RNP binding motifs, which indicates that splicing, transport, or other RNA regulatory steps are involved in the diversification of segmental morphology, Previous studies by Dickson and others (Dickson, B. J., Van Der Straten, A., Dominguez, M. and Hafen, E. (1996). Genetics 142, 163-171) identified spell as a gene that acts downstream of Raf to suppress Raf signaling in a manner similar to the ETS transcription factor Aop/Yan. This raises the intriguing possibility that the Spen RNP protein might integrate signals from both the Raf and Hox pathways.