Structural basis of Drosophila insulin receptor activation by DILP2 hormone

Insulin-related hormones regulate key life processes in the animal kingdom, from metabolism to growth, lifespan and aging, through an evolutionarily conserved insulin and insulin-like hormones signalling axis (IIS). In humans the IIS axis is controlled by insulin, two Insulin-like Growth Factors, two isoforms of the insulin receptor (hIR-A and -B), and its homologous IGF-1R. In Drosophila, this signalling engages seven insulin-like hormones (DILP1-7) and a single receptor (dmIR) that follows the blueprint of hIR/hIGF-1R. This report describes two cryo-EM structures of the dmIR ectodomain dmIR-ECD:DILP2 complex, revealing their structural homology with dmIR:DILP5 complex. The high excess of DILP2 yielded two dmIR-ECD complexes in asymmetric conformations, similar to that observed in some complexes of hIR and in the dmIR-ECD:DILP5 complex. This stoichiometric and structural heterogeneity, yielding one- and two-DILP2:receptor complexes – were not observed in DILP5:dmIR-ECD assembly. Also, the resistance of dmIR-ECD to form more DILP2 saturated complexes, despite very high excess of this hormone, suggest that the specificities of DILPs may lie in their k _on/k _off kinetic parameters. This work expands understanding of the dmIR conformational flexibility, suggesting also that insect dmIR follows more hIR rather than hIGF-1R receptor signal transduction pattern induced by various DILPs.