ABSTRACT
Most biological functions are mediated by protein interactions. These interactions can be physical, such as when two proteins form a complex, or “logical,” such as when one or more proteins control the behavior of one or more other proteins without physical interaction. Examples of physical interactions are stable complexes, in which the functional unit is formed by more than one protein chain, as in the case of the glycogen phosphorylase enzyme, and transient associations, in which the protein chains are stable by themselves but can also interact to transmit a signal or as a response to external conditions. In logical interactions, one protein affects another protein by, for example, regulating its expression or changing the concentration of a factor that, in turn, is sensed by the target protein. The two modes of interaction are not exclusive. The same proteins can interact both physically and logically.
