nonreceptors

Nonreceptors are proteins and enzymes that participate in cellular signaling but do not function as cell-surface receptors. Unlike receptors, which bind ligands and typically initiate signaling by undergoing conformational changes and transmitting signals across the cell membrane, nonreceptors execute intracellular actions in response to upstream cues. They can act as enzymes, phosphatases, kinases, phospholipases, or scaffolds that regulate signal propagation, metabolism, gene expression, or cytoskeletal organization. They may be located in the cytoplasm, nucleus, or at intracellular membranes and are often activated following recruitment to activated receptors or by second messengers.

A notable subclass is nonreceptor tyrosine kinases (NRTKs). These enzymes reside in the cytoplasm or nucleus and do not span the membrane. After their respective receptors are activated, NRTKs phosphorylate target proteins to propagate signals. Prominent examples include the Src family kinases (Src, Yes, Fyn, Lyn), Syk, Abl, and the JAK family, which associate with receptor cytoplasmic domains.

Therapeutic targeting frequently focuses on nonreceptors, such as kinases or phosphatases, to modulate aberrant signaling in cancer and other diseases. The term nonreceptors thus helps distinguish these intracellular signaling proteins from membrane-anchored receptors.

Because the phrase nonreceptor can be used in different contexts, definitions may vary; some use the term strictly for nonreceptor tyrosine kinases, while others apply it more broadly to intracellular signaling proteins lacking receptor function.