Proteins, including membrane proteins, also play critical roles in signal transduction, or relay. Signals can include hormones, ions, environmental changes such as odors or light, or mechanical disturbances such as stretching. A hormone is a small molecule released by one cell in the body to influence the behavior of another. A hormone exerts its influence by binding to a protein receptor in the target cell either on the membrane or within the cytoplasm. Cells that do not make receptors for a particular hormone are not susceptible to its effects. Adrenaline and testosterone are examples of hormones that illustrate two major modes of hormone action.
Adrenaline binds to a membrane-spanning receptor that projects both to the outside and the inside of the cell. The binding of adrenaline to the exterior portion changes the shape of the receptor, which in turn sets in motion other changes within the cell. The result is the production of a molecule called cyclic AMP (adenosine monophosphate), another form of the adenosine nucleotide. This "second messenger" binds to a variety of enzymes within the cell, activating them and leading to production of a variety of products. The exact set of enzymes turned on by cyclic AMP and the exact set of consequences depend on the particular cell. Kidney cells, for instance, increase their permeability to water, while liver cells release sugar into the bloodstream. The unique set of proteins within each cell is determined by the genes it has expressed, which in turn is determined by its own history and the hormones and other influences to which it has been exposed.
Testosterone's effects come on more slowly than adrenaline's, but last much longer. Testosterone passes through the plasma membrane and binds to a receptor in the cytosol. Once this occurs, the receptor-hormone complex is transported to the nucleus. Here, it binds to DNA, altering the rate of gene expression for a wide variety of genes. Thus, testosterone acts as a transcription factor. The prolonged action of testosterone is in part because it stimulates the production of new, long-lasting proteins that alter the cell's function for much longer than the very rapid and short-lived effects of adrenaline.
Cells continually respond to signals, and they influence other cells through the signals they release. Signaling pathways within the cell control the rate of cell division, the development and differentiation of the cell, the secretion of proteins and other molecules, and the response to injury, among many other reactions.
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