Insulin/receptor binding: The last piece of the puzzle?

Abstract

Progress in solving the structure of insulin bound to its receptor has been slow and stepwise, but a milestone has now been reached with a refined structure of a complex of insulin with a "microreceptor" that contains the primary binding site. The insulin receptor is a dimeric allosteric enzyme that belongs to the family of receptor tyrosine kinases. The insulin binding process is complex and exhibits negative cooperativity. Biochemical evidence suggested that insulin, through two distinct binding sites, crosslinks two receptor sites located on each α subunit. The structure of the unliganded receptor ectodomain showed a symmetrical folded-over conformation with an antiparallel disposition. Further work resolved the detailed structure of receptor site 1, both without and with insulin. Recently, a missing piece in the puzzle was added: the C-terminal portion of insulin's B-chain known to be critical for binding and negative cooperativity. Here I discuss these findings and their implications.

Structure of the insulin receptor–insulin complex by single-particle cryo-EM analysis

“…Red: interactions predicted by biochemical analysis 6 , but not seen in the microreceptor structures 7 . Green: new interactions identified in the cryoEM map. * Insulin residues predicted to belong to S2, with no IR partner assigned 6 . …”

“…Light blue: interactions structurally characterized 7 . Red: interactions predicted by biochemical analysis 6 , but not seen in the microreceptor structures 7 . Green: new interactions identified in the cryoEM map.…”

“…Earlier work suggested that these insulin residues interact with the S2 binding site 6 but no IR partner residues were assigned. Interactions are observed between the α-CT helix residues Leu696-Lys703 and IR residues Gly346-Asn349 and Arg372-Tyr374 (L2 subdomain) and Arg498-Leu501 and Val570-Thr571 (FnIII-1 subdomain).…”

“…In those structures insulin directly interacts with the IR L1 subdomain through only a small region of the B-helix, and most of the other IR residues that have been identified as essential for insulin binding (Extended Data Table 1) interact with the α-CT helix. Predicted interactions involving the C-terminal end of the insulin B-chain 6 could not be confirmed because those residues were not resolved in the electron density map 7 . The cryoEM structure confirms the engagement of S1, including the interactions involving the C-terminus of the insulin B-chain which is now resolved.…”

“…Insulin binding to the dimeric ECD triggers kinase domain auto-phosphorylation and subsequent activation of downstream signaling molecules. Biochemical and mutagenesis data have identified two putative insulin binding sites (S1 and S2) 6 . While insulin bound to an ECD fragment containing S1 and the apo ectodomain have been characterized structurally 7,8 , details of insulin binding to the full receptor and the signal propagation mechanism are still not understood.…”