Signal transduction pathways enable cells to act in response to the perception of stimuli in their environment and the integration of external and internal signals by changes in transcriptional activity, metabolism, or other regulatory measures. The proper functioning of these pathways is vital for cell adaptation and survival under varying conditions, as well as for cell differentiation, fate, and death.
Cell signal transduction can be simplistically recognized as three steps: reception, transduction, and induction. Reception entails the binding of a signal molecule (e.g., a hormone) to its specific receptor. Transduction is the process by which, for example, a second messenger is formed in or released into the cytosol, thus amplifying the stimulus and initiating the cell's response to the signal. Induction results in activation of the cellular process.
The study of individual cells and cell lines allows us to identify features of signal transduction, but work on physiological models defines what is relevant in the physiology of a given cell type at a given stage. Indeed, the simplified experimental paradigms of conventional cell biology systems used for the study of signal transduction are a double-edged sword. At each level from the in vivo microscopic study of protein–protein interactions (PPIs) in single cells to the precise molecular structural definition of proteins by X-ray crystallography, what we gain in precision, rigor, and definition we may lose in relevant biology.