Effector (biology)

In biology, an effector is a general term that can refer to several types of molecules or cells depending on the context: Small molecule effectors A small molecule that selectively binds to a protein to regulate its biological activity can be called an effector. In this manner, effector molecules act as ligands that can increase or decrease enzyme activity, gene expression, influence cell signaling, or other protein functions. An example of such an effector is oxygen, which is an allosteric effector of hemoglobin - oxygen binding to one of the four hemoglobin subunits greatly increases the affinity of the rest of the subunits to oxygen. Certain drug molecules also fall into this category - for example the antibiotic rifampicin used in the treatment of tuberculosis binds the initiation σ factor subunit of the bacterial RNA polymerase, preventing the transcription of bacterial genes. The term can also be used to describe small molecules that can directly bind to and regulate the expression of mRNAs. One example for such an effector is guanine, which can be recognised by specific sequences (known as riboswitches) found on mRNAs, and its binding to those sequences prevents the translation of the mRNA into a protein. See also: purine riboswitch. Protein effectors An effector can also be used to refer to a protein that is involved in cellular signal transduction cascades. Such an example are RAS effector proteins, which are all able to bind RAS.GTP, but trigger different cell pathways upon doing so - such as the Ras-Raf-MEK-ERK pathway, the PI3K pathway or several others. An effector hormone is a hormone that acts on a particular tissue - an example of such a hormone is thyroxine (T4), which regulates metabolism in many tissues throughout the body. Antibody Effectors are effectors involved with the production and secretion of molecules involved in pathogen defense, such as Immunoglobulin. Many antibodies then act as effector molecules for the immune system of the organism, typically as enzyme activators.