Activation loop conformation of the insulin receptor and its relationship with insulin receptor regulation

The insulin receptor regulates metabolic activity through its tyrosine kinase activity. The tyrosine kinase activity of the insulin receptor becomes activated after insulin binding causes the receptor to autophosphorylate in trans. Prior to insulin stimulation, maintaining the receptor in its basal, unphosphorylated state is important for keeping the kinase activity latent and preventing constitutive signaling. While many other receptor tyrosine kinases prevent their trans-autophosphorylation by existing in the cell membrane as monomers, which can be dimerized to form an activated receptor through ligand binding, the insulin receptor is a pre-existing dimer and cannot use typical ligand induced dimerization to prevent constitutive autophosphorylation and signaling; some other regulatory mechanism must be used to maintain the basal state of the insulin receptor.; One possibility for such a regulatory mechanism is introduced by the crystal structure of the unphosphorylated, basal state insulin receptor kinase domain. In the crystal structure, the activation loop of the kinase domain is observed lying in and occluding the catalytic cleft, preventing the entry of nucleotide and peptide substrates and sequestering a tyrosine residue believed to be the initial target of trans-autophosphorylation. This intrasteric inhibition renders the kinase latent as an enzyme, and as a substrate for the autophosphorylation reaction. It appears that intrasteric inhibition of the kinase and the resulting dual latency could keep the insulin receptor in its basal state.; In this work we present the results of studies designed to investigate how prevalent the intrasterically inhibited conformation of the insulin receptor kinase domain is in solution and how it could be relieved to allow for autophosphorylation and activation. Based on our finding that the binding of adenine nucleotides to the kinase domain relieves intrasteric inhibition, we investigated whether intrasteric inhibition was necessary to maintain the basal state of the insulin receptor under physiological conditions and whether the conformation of the activation loop responsible for intrasteric inhibition and latency might be linked to insulin binding affinity at the extracellular domains of the receptor. Collectively, these studies attempt to describe whether the conformation of the activation loop plays a role in the regulation of the insulin receptor.