Which of the following is TRUE concerning the interaction between the insulin receptor and IRS-1:

• A. The IRS protein phosphorylates the insulin receptor.
• B. The β-subunits of the receptor bind to insulin, where each subunit has a separate insulin binding site.
• C. Each β-subunit acts as a serine kinase and auto-phosphorylates the other subunit.
• D. IRS is phosphorylated by the tyrosine kinase domains in the β-subunits of the insulin receptor.
• E. IRS contains phosphorylated threonine residues that directly bind to insulin.

Answer: (D)

Verified step by step guidance

1. Understand the role of the insulin receptor: The insulin receptor is a transmembrane receptor that is activated by insulin. It is composed of two α-subunits and two β-subunits.
2. Identify the function of the β-subunits: The β-subunits of the insulin receptor have intrinsic tyrosine kinase activity, which means they can phosphorylate tyrosine residues on target proteins.
3. Clarify the role of IRS-1: IRS-1 (Insulin Receptor Substrate 1) is a key signaling molecule that becomes phosphorylated on tyrosine residues by the activated insulin receptor.
4. Examine the interaction between the insulin receptor and IRS-1: Upon insulin binding, the β-subunits of the receptor undergo autophosphorylation, which activates their kinase activity. This leads to the phosphorylation of IRS-1 on specific tyrosine residues.
5. Conclude the correct statement: The correct interaction is that IRS-1 is phosphorylated by the tyrosine kinase domains in the β-subunits of the insulin receptor, facilitating downstream signaling pathways.