Where can the enzymatic domain of enzyme-linked receptors be located?

• A. Part of the receptor itself
• B. Part of a cytosolic protein recruited to the receptor
• C. Located in the extracellular matrix
• D. Part of the receptor itself or part of a cytosolic protein that is recruited to the receptor in response to receptor activation

Answer: (D)

Enzyme-linked receptors propagate signals through a catalytic activity that can either be built into the receptor itself or provided by a cytosolic enzyme that is recruited when the receptor is activated. In receptor tyrosine kinases, the enzymatic domain sits in the receptor’s cytoplasmic tail, so ligand binding induces dimerization and autophosphorylation that triggers downstream signaling. In other receptor families, like some cytokine receptors, there is no intrinsic enzymatic activity; instead, a cytosolic kinase such as JAK associates with the receptor and becomes activated upon receptor engagement, phosphorylating the receptor and signaling proteins. Therefore, the enzymatic domain can be located either within the receptor itself or in a cytosolic protein recruited to the receptor after activation. An extracellular matrix location wouldn’t house this signaling enzymatic activity.