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Once insulin has bound the α subunit of the insulin receptor, the β subunit of the receptor can bind to an insulin receptor substrate (IRS) protein, which acts as a membrane scaffold. What events happen next that enable the β subunit and the IRS protein to bind together?

a. Binding of insulin induces conformational changes in the β subunits, which expose their serine/threonine kinase active sites, enabling them to autophosphorylate on threonine residues. These P-threonine residues are recognized by the Src homology 2 sites (SH2 domain) present in an IRS protein.
b. Conformational changes in the β subunits occur as a result of insulin binding to the α subunit. These expose the tyrosine kinase active sites in the β subunits, which next phosphorylate IRS proteins on select tyrosine residues. Phosphotyrosine binding sites (PTB domain) found in the β subunit of the receptor then bind to the P-tyrosine residues in the IRS protein.
c. Binding of insulin induces conformational changes in the β subunits, which enable them to autophosphorylate on tyrosine residues. These P-tyrosine residues are recognized by the phosphotyrosine binding sites (PTB domain) present in an IRS protein.
d. Conformational changes in the β subunits occur as a result of insulin binding to the α subunit. These expose serine/threonine kinase active sites in the β subunits, which next phosphorylate IRS proteins on select threonine residues. Phosphothreonine binding sites (PTB domain) found in the β subunit of the receptor then bind to the P-threonine residues in the IRS protein.