Steffen Runge

Crystal structure of the ligand-bound glucagon-like peptide-1 receptor extracellular domain.

The glucagon-like peptide-1 receptor (GLP-1R) belongs to Family B1 of the seven-transmembrane G protein-coupled receptors, and its natural agonist ligand is the peptide hormone glucagon-like peptide-1 (GLP-1). GLP-1 is involved in glucose homeostasis, and activation of GLP-1R in the plasma membrane of pancreatic β-cells potentiates glucose-dependent insulin secretion. The N-terminal extracellular domain (nGLP-1R) is an important ligand binding domain that binds GLP-1 and the homologous peptide Exendin-4 with differential affinity. Exendin-4 has a C-terminal extension of nine amino acid residues known as the “Trp cage”, which is absent in GLP-1. The Trp cage was believed to interact with nGLP-1R and thereby explain the superior affinity of Exendin-4. However, the molecular details that govern ligand binding and specificity of nGLP-1R remain undefined. Here we report the crystal structure of human nGLP-1R in complex with the antagonist Exendin-4(9–39) solved by the multiwavelength anomalous dispersion method to 2.2Å resolution. The structure reveals that Exendin-4(9–39) is an amphipathic α-helix forming both hydrophobic and hydrophilic interactions with nGLP-1R. The Trp cage of Exendin-4 is not involved in binding to nGLP-1R. The hydrophobic binding site of nGLP-1R is defined by discontinuous segments including primarily a well defined α-helix in the N terminus of nGLP-1R and a loop between two antiparallel β-strands. The structure provides for the first time detailed molecular insight into ligand binding of the human GLP-1 receptor, an established target for treatment of type 2 diabetes.

H19 RNA Binds Four Molecules of Insulin-like Growth Factor II mRNA-binding Protein

H19 RNA is a major oncofetal 2.5-kilobase untranslated RNA of unknown function. The maternally expressedH19 gene is located 90 kilobase pairs downstream from the paternally expressed insulin-like growth factor II(IGF-II) gene on human chromosome 11 and mouse chromosome 7; and due to their reciprocal imprinting and identical spatiotemporal expression, it is assumed that the two genes are functionally coupled. Here we show that human H19 RNA contains four attachment sites for the oncofetal IGF-II mRNA-binding protein (IMP) with apparentK d values in the 0.4–1.3 nm range. The multiple attachment sites are clustered within a 700-nucleotide segment encoded by exons 4 and 5. This 3′-terminal segment targets H19 RNA to lamellipodia and perinuclear regions in dispersed fibroblasts where IMP is also localized. The results suggest that IMP participates in H19 RNA localization and provides a link between the IGF-II and H19genes at post-transcriptional events during mammalian development.