Fig. 7. srt encodes an evolutionarily conserved multi-transmembrane protein. (A) Injection of srt mRNA into non-paternally rescued germline clone embryos rescues the segmentation defect (right). The buffer-injected control shows the null srt phenotype. (B) Injection of a 543 bp srt dsRNA into GFP⁺ paternally rescued srt embryos yields a cuticle phenotype (right) almost as severe as the buffer-injected non-paternal rescued embryo in A and Fig. 1C. (C) The srt genomic region located at 68A9 is a gene-dense region. The srt ORF is flanked closely by Alg10 (left) and shares its 5 ' proximal region with CG7616 (right; www.flybase.org). The srt ORF is composed of two or three exons to yield two splice variants. (D) One possible topological model for the protein encoded by srt: the luminal/extracellular region, above; the cytoplasmic region, below. Four putative transmembrane stretches are shown. The 7E4 mutation in srt results in a nonsense mutation in the fourth transmembrane span in our model. The location of the splice variant in the intracellular loop and possible sites of N-linked glycosylation are shown. (E) Sequence alignment of Drosophila srt B isoform (NCBI accession: NP_729681) with its homologs from C. elegans (NP_001022275), X. laevis (AAH81130) and human (NP_079187). The four transmembrane domains shown in D are underlined in black. Four possible additional transmembrane domains are underlined in gray. The asterisk under the last transmembrane marks the location of the srt^7E4 mutation.