Introduction to Signaling Networks Biophysics 702, February 2012 Jonathan P Butchar

Intracellular and Intercellular signaling General Signaling Concepts Types of Signaling Initial Signaling Components –Receptors –Ligands Downstream Signaling Components –Kinases –Phosphatases –Adaptors & Effectors

General Signaling Concepts

Figure 15-8 Molecular Biology of the Cell (© Garland Science 2008) Signaling is multifaceted and dynamic Different outcomes, even from a single signal –e.g., Strength of signal Convergence of signals Crosstalk between pathways Cells can adapt over time –dampened or heightened responses Signals can be amplified or dampened via feedback Primary versus secondary responses

Types of Signaling

Numerous ways to signal Contact-dependent Autocrine / Paracrine Endocrine Direct Transmission (gap junctions)

Initial Signaling Components Receptors and Ligands

Receptors Cell surface Intracellular

Ligands Peptides / Proteins Steroids Nucleotides Fatty Acids Gases Mechanical Forces …etc

How are these signals transmitted? Ion fluxes G-protein activation Enzyme activation (e.g., Phosphorylation)

Figure 15-53a Molecular Biology of the Cell (© Garland Science 2008) Example: Receptor Tyrosine Kinases

Downstream Signaling Components Kinases Phosphatases Adaptors Effectors

What molecules transmit the signals? Kinases add phosphate groups Phosphatases remove these phosphates Guanosine Tri-Phosphate / GDi-P cycles drive many signaling cascades Adaptors help bring necessary molecules together Effectors generate the responses –commonly gene transcription

Figure 15-18a Molecular Biology of the Cell (© Garland Science 2008) Kinases and phosphatases e.g., Receptor Tyrosine Kinase: a receptor AND a kinase

Figure 15-18b Molecular Biology of the Cell (© Garland Science 2008) Guanosine TriPhosphate and Guanosine DiPhosphate e.g., G-protein coupled receptor

Figure Molecular Biology of the Cell (© Garland Science 2008) Trimeric G-proteins 3 subunits –α, β, γ

…another view, from Wikipedia Activated when GTP replaces GDP, inactivated when GTP is auto-hydrolyzed to GDP

Figure Molecular Biology of the Cell (© Garland Science 2008) Monomeric G-proteins Most well-known is Ras –small GTPase –downstream Raf binds only GTP-Ras, which phosphorylates and hence activates Raf GTPase-activating protein Guanine nucleotide exchange factor

Figure Molecular Biology of the Cell (© Garland Science 2008) Adaptor and Scaffold proteins have characteristic domains SH2 binds phosphotyrosine SH3 binds proline- rich domains PH binds phosphoinositides Proline-rich

How proper localization is achieved Association at plasma membrane Scaffolding proteins Direct receptor binding

Figure Molecular Biology of the Cell (© Garland Science 2008) Signaling Example: from G-proteins to gene transcription Activated receptor Activated G-protein Activated Protein Kinase A The Effector, an activated transcriptional modulator

Lines are blurry at times Some membrane-bound receptors can go to the nucleus and regulate gene transcription –Both a receptor and an effector Phosphorylation can sometimes deactivate rather than activate a protein

Summary Cells and groups of cells possess mechanisms to generate and respond to signals Signaling can be autocrine, paracrine, endocrine, synaptic, electrical or mechanical Receptors sense numerous types of stimuli and begin cascades that lead to cellular responses Observed responses represent an integration of numerous stimuli, both past and present