IPs subunits are not so clearly defined; they likely participate in transferring the signal transduction from the occupied receptor to the Gas and Gai subunits. There are also comparable forms of Gs that activate the phosphodiesterase and membrane channels. Table 1-7 summarizes the signal transduction activities of G proteins.

Figure 1-35 presents a model of how GTP as an activating ligand and the intrinsic GTPase enzymatic activity of the as or «¡-subunits are able to function as


Ch) g protein Receptor / Enzyme Extracellular side

Cell membrane

Intracellular side

ATP cAMP (second messenger)

Inactive protein kinase

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