Dynamic control of signaling by modular adaptor proteins

Adaptor proteins are composed exclusively of domains and motifs that mediate molecular interactions, and can thereby link signaling proteins such as activated cell-surface receptors to downstream effectors. Recent data supports the notion that adaptors are not simply coupling devices that hard-wire successive components of signaling pathways. Rather, they display highly dynamic properties that direct the flow of information through signaling networks. The binding activity of adaptors can be regulated by conformational reorganization, and by the cooperative association of domains within the same adaptor. Furthermore, an individual adaptor can deliver different outputs by utilizing distinct combinations of binding partners. Adaptors can also control the oligomerization of receptor signaling complexes, and the subcellular location and duration of signaling events, and act as coincidence detectors to enhance specificity in cellular responses.

Introduction

The term ‘signaling adaptor’ was originally used to describe a class of proteins that are composed exclusively of interaction domains and binding motifs, and that can link cell-surface receptors to intracellular targets, which in turn regulate specific downstream signaling pathways [1]. In principle, such adaptors have a domain that selectively recognizes an activated receptor, and one or more domains that recruit cytoplasmic effectors, as in the case of the prototypic SH2/SH3 adaptors Crk and Grb2 [2]. For example, Grb2 has a single SH2 domain that binds preferentially to phosphorylated YXN sites on activated receptor tyrosine kinases (RTK), and two flanking SH3 domains that engage proteins such as Sos (a Ras guanine nucleotide exchange factor) and the Gab1/2 docking proteins, which in turn become phosphorylated and recruit SH2-containing targets such as phosphatidylinositol (PtdIns) 3′-kinase and the Shp2 tyrosine phosphatase [3, 4]. Grb2 can therefore couple RTKs to pathways, such as the Ras-MAP kinase (Ras-MAPK) and PtdIns 3′-kinase pathways, involved in growth, proliferation and differentiation. Indeed Grb2 acts in the very early mouse embryo (at embryonic day 3.5) to couple RTK signals to the Ras-MAPK pathway, which induces expression of Gata6, a transcription factor required for the formation of the primitive endoderm (PE) and thus production of extraembryonic endoderm layers of the visceral and parietal yolk sac [5^•]. This Grb2-mediated pathway in PE progenitors also suppresses expression of the homeodomain protein Nanog, which specifies the epiblast progenitors that give rise to the fetus. As a consequence, Grb2^−/− embryos ectopically express Nanog and fail to produce Gata6; they therefore lack primitive endoderm lineages and arrest at E4.5 [6].

In the case of Grb2, structural analysis has suggested that the SH2 and SH3 domains are unlikely to be in direct contact, and the multiple domains in this adaptor may therefore effectively function as beads on a string [7]. As previously argued [8], and discussed below, this simple situation is likely the exception rather than the rule.

Subsequent data have indicated that adaptor proteins are used very widely in signaling from different types of receptors at the cell surface, both in coupling receptors to their proximal targets and in directing the flow of information in the intracellular networks that control cellular responses. In addition, adaptors are important in pathways activated by internal signals, such as DNA damage [9]. Recent results have also suggested that the binding properties of adaptors can themselves be dynamically regulated, for example by intramolecular interactions. Furthermore, the association of adaptors with their binding partners can provide cooperative effects and gating functions that go well beyond the simple linking of upstream activators and downstream targets. It is also becoming apparent that an adaptor may provide diversity in signaling by engaging distinct combinations of activators and targets in different cell types, or even at different locations in the same cell. Indeed the trafficking of signaling proteins is itself dependent on adaptor proteins, such as those associated with the endocytic machinery and protein sorting in endosomes [10, 11, 12]. Taken together, these findings indicate that adaptors are not simply hard-wired components of signaling pathways, but play a highly dynamic role in the cell's response to fluctuating external and intrinsic cues. Here, I discuss recent analysis of signaling adaptors and scaffold proteins.