Molecular mechanisms in signal transduction at the membrane

Abstract

Signal transduction originates at the membrane, where the clustering of signaling proteins is a key step in transmitting a message. Membranes are difficult to study, and their influence on signaling is still only understood at the most rudimentary level. Recent advances in the biophysics of membranes, surveyed in this review, have highlighted a variety of phenomena that are likely to influence signaling activity, such as local composition heterogeneities and long-range mechanical effects. We discuss recent mechanistic insights into three signaling systems-Ras activation, Ephrin signaling and the control of actin nucleation-where the active role of membrane components is now appreciated and for which experimentation on the membrane is required for further understanding.

Figure 1

Activation of PKC at the membrane. (a) Inactive PKC is associated with heat-shock protein 90 (HSP90) at the membrane. (b) The activation of cell-surface receptors results in the production of DAG in the membrane and an increase in cytoplasmic Ca²⁺. PKC is primed for activation by phosphorylation, which is carried out by two protein kinases, phosphoinositide-dependent kinase 1 (PDK1) and mammalian target of rapamycin complex (mTORc, not shown), which release it from HSP90 and the membrane. These two signals, along with PIP₂ in the membrane, result in the recruitment of PKC to the membrane and its allosteric activation. Schematic diagrams are adapted from previous work^,.

Figure 2

Self-organizing spiral waves formed by Min proteins on the membrane surface. Left, only labeled MinE is shown (MinD, 1 mM; MinE, 1 mM); right, labeled MinD and MinE are shown (MinD, 1 mM; MinE, 1 mM). Figure is adapted from previous work.

Figure 3

The activation of Ras by SOS. (a) In the textbook model for Ras activation, activated growth factor receptors recruit SOS to the membrane, where it finds and activates Ras. (b) Membranes enhance the binding affinity between two proteins only if both are localized to the same membrane. (c) The presence of the allosteric binding sites for Ras on SOS greatly increases the activity of the SOS_cat domain by recruiting SOS to the membrane. The specific activity of the SOS_cat domain is dependent on the Ras surface density. (d) The regulatory domains of SOS block the allosteric Ras binding site and prevent uncontrolled Ras activation by SOS. Activation involves the coordinated action of phospholipids at the membrane as well as receptor recruitment of SOS. The regulatory domains are destabilized by Noonan syndrome mutations, leading to constitutive activation. PA, phosphatidic acid; H, histone domain. Rem and Cdc25 are the catalytic modules of SOS. Panels c and d are adapted from previous work.

Figure 4

EphA2 spatial mutation experiment. (a) Schematic of a hybrid live cell–supported membrane junction between a live human epithelial cell expressing EphA2 and a patterned supported membrane expressing ephrin-A1. Physical barriers on the substrate block the lateral mobility of supported membrane molecules and associated molecules in the living cell. (b) Spatial mutation of EphA2– ephrin-A1 complexes alters recruitment of A disintegrin and metallopeptidase 10 (ADAM10). BF, bright field microscopy. Figure is adapted from previous work.

Figure 5

Model for activation of the WAVE complex. The WAVE complex is intrinsically inactive. The first step in activation is phosphorylation, which primes the complex for interaction with the membrane. Binding to acidic phospholipids, particularly PIP₃ and Rac-GTP, is required for activation. Figure is modified from previous work.