G protein-coupled receptors (GPCRs) are a large family of cell surface receptors that respond to various extracellular signals. Which of the following statements is INCORRECT?

1
All GPCRs are transmembrane proteins with seven membrane-spanning helices.
2
Binding of ligand to GPCR causes the receptor to activate G proteins by facilitating the exchange of GDP for GTP on the G protein.
3
GPCRs can directly phosphorylate downstream signaling proteins to elicit cellular responses.
4
GPCRs are involved in numerous physiological processes, including vision, taste, and smell.

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