Signalling Scaffolds and Local Organization of Cellular Behaviour
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REVIEWS Signalling scaffolds and local organization of cellular behaviour Lorene K. Langeberg and John D. Scott Abstract | Cellular responses to environmental cues involve the mobilization of GTPases, protein kinases and phosphoprotein phosphatases. The spatial organization of these signalling enzymes by scaffold proteins helps to guide the flow of molecular information. Allosteric modulation of scaffolded enzymes can alter their catalytic activity or sensitivity to second messengers in a manner that augments, insulates or terminates local cellular events. This Review examines the features of scaffold proteins and highlights examples of locally organized groups of signalling enzymes that drive essential physiological processes, including hormone action, heart rate, cell division, organelle movement and synaptic transmission. This Review is dedicated to the memory of phosphoprotein phosphatases; and the temporal con- Tony Pawson, our friend, our colleague and a trol of rapid signalling events, such as muscle contrac- pioneer in this field. tion, synaptic transmission and retrograde transport of organelles along microtubules. A well-worn adage, rediscovered by successive genera- At the dawn of the twenty-first century, many of these tions of researchers, is that ‘the longer a topic is inves- enzyme-binding proteins had been classified as adaptor, tigated, the more complex the questions become’. This docking, anchoring or scaffold proteins14. Although it is is certainly true for biomedical scientists, who grapple cumbersome and confusing, this arbitrary terminology with the ever-accumulating molecular details of cell is now firmly entrenched in the signalling community. regulation. Within the perceived chaos of the cell, local However, despite its utility, it is often difficult to assign organization of signalling enzymes guarantees the fidel- an individual protein to a single class. Therefore, for the ity of information processing. Breakdown of this molec- purposes of this Review, we propose that adaptor proteins ular order can result in disease. The concept of protein are soluble proteins that contain several modular protein- scaffolds as control centres for the integration and interaction domains within their structure (FIG. 1a). dissemination of subcellular information has evolved Prototypical examples include growth factor receptor- considerably during the past 25 years. bound protein 2 (GRB2) and SHC1, both of which are The protein scaffold model originates from three composed of SRC homology 2 (SH2) and SH3 domains conceptual and technical breakthroughs in the late and enable selective and simultaneous recruitment of 1980s and early 1990s. These were: the realization that several signalling elements15,16 (FIG. 1a). Similarly, docking protein-interaction modules are the building blocks proteins are composed of modular protein-interaction of macromolecular assemblies1–6; the use of yeast two- domains, with a distinguishing feature being that they hybrid and proteomic analyses as a universal means to sequester signalling components at the cell membrane Howard Hughes Medical systematically identify protein–protein interactions7,8; next to an activating receptor (FIG. 1b). Proteins emblem- Institute and Department of and innovative genetic screens in lower organisms that atic of this class include insulin receptor substrate 1 (IRS1), Pharmacology, (FIG. 1b) University of Washington, uncovered functional relationships between signal- IRS2, IRS3 and IRS4 , which help to transmit sig- 9–11 1959 NE Pacific Street, ling elements . As investigators from different disci- nals from insulin to various intracellular PI3K–AKT and Seattle, Washington 98195, plines exploited these advances, it was soon recognized MAPK cascades; and fibroblast growth factor receptor USA. that a substantial proportion of human genes encode substrate 2α (FRS2α), which links its receptor to several Correspondence to J.D.S. enzyme-binding proteins12,13. We now know that these downstream signalling pathways17–19. e‑mail: [email protected] doi:10.1038/nrm3966 ancillary signalling elements participate in the organi- In contrast to the aforementioned examples of adaptor Published online zation of MAPK cascades; the subcellular targeting proteins and docking proteins, the delineation between 18 March 2015 of second-messenger-regulated protein kinases and anchoring and scaffold proteins is much less clear12,20,21. 232 | APRIL 2015 | VOLUME 16 www.nature.com/reviews/molcellbio © 2015 Macmillan Publishers Limited. All rights reserved REVIEWS a Membrane Adaptor b Insulin Docking Pseudokinases and pseudophosphatases receptor protein (GRB2) receptor protein (IRS1) Of the 544 protein kinases (G. Manning, personal com- Plasma Plasma PH munication) that constitute the human kinome, 55 are membrane SH3 RAS membrane P PTB P SH2 currently known to be pseudokinases — kinase-like SH3 SOS P proteins that lack the key residues essential for catalytic PI3K 22–24 P P activity (BOX 1). Rather than functioning enzym Recruited P P P P atically, pseudokinases and their counterparts, pseudo signalling PDK1 AKT enzymes phosphatases, regulate phosphorylation events by Recruited signalling enzymes functioning as allosteric modulators of other signalling enzymes25,26. Another important function of pseudo c d Ion Plasma e kinases and pseudophosphatases is in kinase scaffolding, Signal channel membrane Effector Coordinated regulation either directly by acting as kinase scaffolds or indirectly A C A B of an effector by functioning together with other scaffold proteins. A As a result, these seemingly inert signalling elements are B Signalling increasingly viewed as integral components of signal- cascade C ling pathways. Although there is considerable interest in C B the propagation of oncogenic signals by receptor tyro Scaffold protein sine kinase-like pseudokinases — such as the epidermal Focused enzyme activity growth factor (EGF) receptor family member HER3 (also Figure 1 | Properties of adaptor, docking and scaffold proteins. The figure depicts 27,28 Nature Reviews | Molecular Cell Biology known as ERBB3) — we limit our discussion here to the distinguishing features of signal-organizing proteins and of scaffold proteins. serine/threonine kinase-like pseudokinases. Adaptor proteins, such as growth factor receptor-bound protein 2 (GRB2) (part a), and membrane-associated docking proteins, such as insulin receptor substrate 1 (IRS1) Pseudokinases as scaffold proteins. One exciting new (part b), are both composed of protein-interaction modules that recruit signalling concept is that the activation of certain serine/threonine enzymes near transmembrane receptors21. Essential features of scaffold proteins (shown in green) include the ability to hold in place successive members of a signalling kinases requires the formation of a larger complex cascade (part c), focus enzyme activity at a particular site of action (part d) and provide in which a pseudokinase functions cooperatively as a molecular platform for the coordinated regulation of a particular effector protein by a scaffold protein (FIG. 2a,b). Liver kinase B1 (LKB1; signal transduction and signal termination enzymes (part e). Phosphate groups are also known as STK11) is a tumour suppressor protein depicted as orange circles. PDK1, phosphoinositide-dependent kinase 1; PH, pleckstrin kinase that regulates cellular energy status, cell pro homology domain; PTB, phosphotyrosine-binding domain; SH2, SRC homology 2; liferation and cell polarity29–31. A defining feature of SOS, son of sevenless. LKB1 is that it is not activated by the phosphorylation of its A loop, but instead it is induced into the active state through interaction with the pseudokinase STRAD32 In fact, these terms have been used interchangeably to (FIG. 2a). Structural studies have shown that STRAD describe the same molecule; in this Review, we use the adopts an active kinase-like conformation through its term scaffold protein. Here, we identify three features that interactions with nucleotides and with the horseshoe- are often attributable to this burgeoning class of signal- shaped calcium-binding protein MO25 (also known as organizing proteins (FIG. 1c–e). First, they can be multiva- CAB39)33,34. The closed conformation of the ATP-bound lent binding proteins that hold members of a transduction STRAD engages LKB1, and the concomitant binding cascade in place to optimize signal relay (FIG. 1c). Second, of MO25 to LKB1 stabilizes its active conformation35 they can be non-catalytic organizational elements that (FIG. 2a). These studies have elucidated distinct regulatory focus enzyme activity at a particular site of action (FIG. 1d). roles for each component of the STRAD–LKB1–MO25 Third, they can be structural components that provide a ternary complex. molecular platform for the recruitment of signal trans- duction and signal termination enzymes to enhance the Pseudokinases with multiple functions. Although many bidirectional control of cellular processes (FIG. 1e). pseudokinases have been identified as allosteric modu- As these three features are generic characteristics lators or scaffold proteins, both properties often reside of this expanding protein class, it is important to note within the same protein. For example, kinase suppressor that individual scaffold proteins may only fulfil one or of RAS (KSR) is a multifunctional binding protein that two of these functional roles (FIG. 1c–e). Recent technical brings together and modulates members of the conven- advances are