Final answer:
Before phosphorylation of tyrosine residues on the insulin receptor can occur, the receptor dimerizes upon insulin binding. This enables the kinase domains to autophosphorylate, which is essential for downstream signaling, including the activation of substrates like IRS-1.
Step-by-step explanation:
The hormone insulin specifically binds to the insulin receptor, which is a type of receptor tyrosine kinase. The question refers to a step that occurs before phosphorylation of tyrosine residues in this signaling pathway. Among the options provided, the binding of insulin leads to autophosphorylation of the receptor, which is a prerequisite for further downstream signaling events, including phosphorylation of tyrosine residues on substrates. Before tyrosine residues on the insulin receptor itself become phosphorylated (autophosphorylated), the receptor must dimerize upon insulin binding. This dimerization brings the kinase domains of the receptor into close proximity, enabling them to phosphorylate each other's tyrosine residues.
The steps following this initial binding and dimerization include activation of substrate proteins, such as IRS-1, which further propagate the signal leading to effects like increased glucose uptake via the Glut4 transporter. It should be noted that the activation of G-proteins, formation of cAMP, and opening of ion channels are steps involved in other types of cell signaling pathways and not the initial steps in insulin receptor signaling.