Exon 6 of human JAG1 encodes a conserved structural unit

We determined the 1H-NMR solution structure of the polypeptide encoded by exon 6 of JAG1 and spanning the C-terminal region of EGF1 and the entire EGF2. We show that this single, evolutionary conserved exon defines an autonomous structural unit that, despite the minimal structural context, closely matches the structure of the same region in the entire receptor binding module.In eukaryotic genomes, exon and domain boundaries usually coincide. We report a case study where this assertion does not hold, and show that the autonomously folding, structural unit is delimited by exon boundaries, rather than by predicted domain boundaries.The Notch signaling pathway is a highly connected and tightly regulated signal transduction network that drives developmental processes in all metazoans. Notch signaling controls cell lineage decisions in tissues derived from all three primary germ lines: endoderm, mesoderm, and ectoderm thus playing an essential role in organogenesis [1-3].Both receptors and ligands are membrane-bound proteins, which normally restricts signaling to adjacent cells. Jagged-1, one of the five Notch ligands identified in man, is a single pass type I membrane protein with a large extracellular region made of a poorly characterized N-terminal region, a DSL (Delta/Serrate/Lag-2) domain, a series of 16 epidermal growth factor (EGF) tandem repeats, and a cysteine-rich juxtamembrane region (Figure 1). The DSL domain, together with the first two atypical EGF repeats constitutes Jagged-1 receptor binding region [4,5].We previously showed [6] that a peptide corresponding to EGF2 of human Jagged-1 (residues 263–295) cannot be refolded in vitro in the standard oxidative folding conditions used for other EGFs. As exon 6 of the JAG1 gene encodes not only EGF2 but also part of EGF1, we speculated that exon 6 might encode an autonomously folding unit. We thus prepared a longer peptide encompassing the C-terminal part of EGF1 and the entire EGF2 (Figure 1). This peptide, J1ex6