Endocrinology 8 - Receptor Tyrosine Kinases How does insulin signal to those target cells to elicit a response? Structure of Insulin: - A peptide hormone - o Synthesised as a longer proper-peptide chain o Then cleaved into two peptide chains; alpha and beta - Liked together by two disulphide bonds - And an additional disulphide bond within the alpha chain - All insulin molecules have the same: o position of the disulphide chains o N- and C-terminal regions of A chain o C-terminal region of B chain - This structure is very similar across species RTKs - All hormones need a receptor - Insulin - insulin receptor o Exists as a dimer - They have regions that look alike (the intracellular part) - They tend to be receptors for growth factors immunoglobulin- like domain cysteine- rich domain - tyrosine kinase domain SS SS SS kinase insert region H fibronectin-type-III- like domain CYTOSOL plasma membrane EGF insulin NGF receptor FGF receptor receptor, IGF1 receptor receptor Eph receptor PDGF VEGF receptor, MCSF receptor receptor - Several receptors of this family have similar characteristics - All receptors need an extracellular domain - Intracellular domain - catalytic domain o Has kinase activity - Insulin receptor - expressed from a single gene
Insulin Receptor - Single INSR gene are translated into o two mRNA splice variants IR-A and IR-B - translated proteins are then proteolytically cleaved into a and B chains o form homo/heterodimers - Dimers together by a single disulphide link between a and ß chains - And by two disulphide links extending from each a chain a chain disulphide bond ß chain - Usually only one insulin molecule will bind to each receptor dimer - Even though the receptor has multiple insulin binding sites o Each monomer has two bind sites; site 1 & site 2 - Insulin binds to the low affinity sites - site 1 - Then binds to site 2 on the other chain o But interacts with each monomer of that receptor - Receptor is then activated - Induces a conformational change to the receptor structure - Bring these two intracellular domains closer together o Contain kinase activity - So, they phosphorylate each other (autophosphorylation) - Tyrosine kinase - so phosphorylate tyrosine amino acids within the intracellular region - When the receptor is activated: o Increase the activation in the kinase domain o We generate binding sites for other signalling proteins (i.e. signalling cascade) Protein Phosphorylation - Reversible post-translational modification of proteins - Protein kinases add phosphate groups at serine, threonine or tyrosine - Is catalysed by enzymes known as kinases - Removed by phosphatase by hydrolysing the phosphate group - By adding/removing the phosphate group we can the change the proteins activity; o Negatively and positively SH2 Domains - Src Homology 2 domain (Identified within the Src oncoprotein) - Found in a lot of signalling molecules
- Allows proteins to bind to phosphorylated tyrosine residues on other proteins - Found in adaptor proteins - Bring other signalling proteins