(199d) Spatiotemporal Regulation of Multiprotein Complexes Nucleated in Response to Receptor Activation

In receptor-mediated cell signaling, outside-in information transfer occurs because ligand-receptor binding in the extracellular compartment promotes intermolecular binding events in the cell interior mediated by phosphotyrosine-SH2 domain and other types of protein-protein interactions. Textbook representations of this process belie the fact that phosphotyrosine-SH2 domain interactions (and other protein-protein interactions) are reversible and relatively weak, and that phosphotyrosines can be regulated by protein tyrosine phosphatases with relatively small time scales, which presents potentially important alternatives to a static picture of protein complex assembly and persistence. For example, in a linear chain of molecules nucleated by a receptor and held together by phosphotyrosine-SH2 domain interactions, functional subunits of the chain may dissociate and reach parts of the cell inaccessible to the receptor, but the persistence of those subunits will depend upon the kinetics of dissociation and re-binding of constituent proteins and the kinetics of tyrosine dephosphorylation and re-phosphorylation. Here, we explore this concept to understand the ability of the epidermal growth factor receptor (EGFR) to drive and maintain the association of SRC homology 2 domain-containing phosphatase 2 (SHP2) with the adaptor protein GRB2-associated binder 1 (GAB1), a binding event that promotes SHP2 activity. Through a combined experimental and computational analysis, we find that GAB1-SHP2 complexes can be maintained distal from EGFR through many cycles of GAB1-SHP2 dissociation and GAB1 dephosphorylation. Although the GAB1-SHP2 complex is not associated with EGFR, EGFR activity is required to maintain the complex through the action of an intermediary kinase, which effectively amplifies EGFR activity such that GAB1-SHP2 complexes decay more gradually than EGFR phosphorylation after initial receptor activation. This process is sufficiently robust that GAB1-SHP2 complexes are predicted to be present at significant and functionally meaningful concentrations over the entire intracellular length scale. Interestingly, this newly described signaling amplification mechanism does not appear to be utilized downstream of all receptor tyrosine kinases, providing additional complexity to our evolving understanding of the spatiotemporal regulation of receptor-mediated signaling.