Analysis of receptor internalization as a mechanism for modulating signal transduction

The past decade has witnessed a profound explosion of knowledge in the field of signal transduction mediated by receptor tyrosine kinases. Upon binding of cognate extracellular ligands, these receptors interact with various enzymes and other signaling molecules intracellularly. These protein substrates, which are generally freely diffusing residents of the cytoplasm, as well as the predominantly membrane-associated downstream targets that they activate, are now fairly well characterized molecules. Despite this surge in signaling research, the mechanisms that regulate signaling interactions in a dynamic fashion remain poorly understood, particulary in quantitative terms. We have developed a generalized mathematical model describing the relationships among receptor, substrate, and target molecules with the aim of gaining fundamental insights into one suggested control mechanism: endocytic trafficking-the rapid and induced internalization of ligated receptors and subsequent relocation to intracellular trafficking organelles. This model is very well-suited, in particular, for the epidermal growth factor receptor. Our major conclusion is that when downstream signaling does not rely on phosphorylation of the substrate by the receptor, or when the substrate is phosphorylated to the same extent regardless of receptor location, receptor internalization can still affect signaling if the membrane-associated target of the substrate is differentially partitioned between surface and internal membrane environments. Specificity of this target "availability" effect can derive from molecular specificity of the retention mechanisms that drive this partitioning. A second conclusion is then that differences in substrate or receptor phosphorylation stoichiometries, perhaps due to partitioning of other membrane kinases or phosphatases, can provide additional influence on signaling. Whatever the mechanism, the total observed activity-i.e. the rate of activated target molecules produced per unit time- can be correlated with receptor/ligand binding and trafficking data to determine whether internalization abrogates or amplifies signaling.