All of the following are cell-surface receptors EXCEPT:
A. Transmitter-gated ion channelsThere are three major classes of cell-surface receptors: transmitter-gated ion channels, seven-transmembrane-G-protein-coupled receptors (GPCRs), and enzyme-linked receptors. The superfamily of GPCRs is one of the largest families of proteins, representing over 800 genes of the human genome. Members of this superfamily share a characteristic seven-transmembrane configuration. The ligands for these receptors are diverse and include hormones, chemokines, neurotransmitters, proteinases, inflammatory mediators, and even sensory signals such as odorants and photons. Most GPCRs signal through heterotrimeric G proteins, which are guanine nucleotide regulatory complexes. Thus the receptor serves as the receiver, the G protein serves as the transducer, and the enzyme serves as the effector arm. Enzyme-linked receptors possess an extracellular ligand-recognition domain and a cytosolic domain that either has intrinsic enzymatic activity or directly links with an enzyme. Structurally, these receptors usually have only one transmembrane-spanning domain. Of at least five forms of enzyme-linked receptors classified by the nature of the enzyme activity to which they are coupled, the growth factor receptors, such as tyrosine kinase receptor or serine/threonine kinase receptors, mediate diverse cellular events including cell growth, differentiation, metabolism, and survival!apoptosis. Dysregulation (particularly mutations) of these receptors is thought to underlie conditions of abnormal cellular proliferation in the context of cancer. The following sections will further review two examples of growth factor signaling pathways and their connection with human diseases.