Cellular Signaling and Membrane Proteins

Membrane Protein Function

• Membrane proteins' structure is crucial for moving ions, molecules, and transmitting signals across the membrane.
• Different types of signaling pathways exist, each with essential features. Lipids and enzymes play roles in transmitting signals within the cell.
• Active and passive transport are different types of transport.

Learning Objectives

• Students should be able to explain how membrane protein structure is vital for ion, molecule movement and signal transmission across the membrane.
• Students should be able to describe the essential features of various signaling pathways.
• Identifying the roles of lipids and enzymes in transmitting signals.
• Distinguishing passive, primary and secondary active transport.

5HT2A Serotonin Receptor

• The receptor has varying attached molecules.
• Specific interactions between the hormone and components are highlighted within the receptor.
• The diagram showcases the receptor's structure within the membrane with details of different components.
• Explanations of what happens after the binding process are given.

Cellular Signaling

• All physiological processes involve biochemical interactions and reactions, allowing cellular function and adaptation.
• The binding of signaling molecules to receptors initiates metabolic and gene expression processes.
• Proteins are essential in cellular response regulation.

Signal Transduction

• Signal transduction cascades share common components.
• Binding of signaling molecules in response to physiological stimuli leads to receptor activation.
• Relaying of the primary message to the cell interior occurs via an intracellular secondary messenger.
• Amplification, signal transductions and response, and cascade termination are all part of the cellular processes.

G-Protein-Coupled Receptors (GPCRs)

• GPCRs contain seven transmembrane segments.
• GPCRs are part of a superfamily of membrane proteins.
• Conformational changes release G-proteins, after which the receptors can bind with different ligands.
• Natural and synthetic ligands provide examples of various molecules binding to the receptor.

Characterizing Binding Interactions

• Non-covalent interactions (ionic bonds, hydrogen bonds, van der Waals interactions) influence a molecule's binding affinity.
• Binding affinities characterize and compare interactions between biomolecules, including proteins, ligands, cofactors, substrates, and drugs.
• K_d values represent dissociation constants, with lower values signifying stronger binding. Binding is saturable. Reversible for non-covalent interactions is also important.

β₂-Adrenergic Receptor

• Brian Kobilka and colleagues solved the receptor's structure in inactive and active states.
• Ligand binding induces minor changes in TM5, primarily on the extracellular side.
• A 14 å movement in TM6 relays the signal to the inside of the cell.
• Conformational changes in TM6 activate adenylyl cyclase.
• cAMP activates downstream enzymes like protein kinase A (PKA).

Epinephrine Signaling

• Epinephrine unbinding is the primary method to stop epinephrine signaling.
• This is the most crucial aspect for turning off the signaling pathway.

Ras Proteins

• Ras proteins are a group of small GTPases involved in diverse signaling pathways.
• They regulate cell proliferation and apoptosis, both crucial for cell functions.
• GTP hydrolysis is involved in signaling regulation.
• Uncontrolled GTP hydrolysis can be a cause of diseases

Signaling and Human Health

• Defects in signaling pathways can lead to diseases.
• Post-translational modifications of proteins, and their conformational changes, are vital in regulating these pathways.
• Protein-protein interactions and ligand-receptor interactions are important in activating and inactivating the signaling pathway.
• Understanding protein structures is important to understand their function and drug design can target proteins to modulate cellular responses

Two Other Important Types of Signaling

• Enzyme-linked receptors, such as tyrosine kinases, often have a single transmembrane segment (e.g., insulin receptor).
• Activation of enzyme-linked receptors leads to autophosphorylation or phosphorylation by tyrosine kinases.
• Examples mentioned are insulin, epidermal growth factor (EGF), Jak/STAT proteins, inositol triphosphate (IP3) pathways and diacylglycerol (DAG).
• Phospholipid-mediated signalling involves phospholipase breakdown of phospholipids to initiate various downstream signalling events like calcium release.

Hormone vs. Hormone

• Insulin and epinephrine compete as hormones for various cellular responses.
• Phosphorylation of insulin receptor substrate (IRS)-1 and activation of the pathway leads to ẞ-adrenergic receptor phosphorylation by PKB.
• Internalization and degradation of GPCR pathways stops signalling via PKB and terminate the signals.

Membrane Transport

• Small, uncharged, or lipophilic molecules cross cell membranes via passive diffusion, which is dependent on substrate concentration.
• Cell transport is crucial for life. Nutrients are brought in and metabolic waste is removed out
• Inorganic ions move in and out of the cell via various transport mechanisms.
• Integral membrane proteins facilitate facilitated diffusion and active transport, with some requiring ATP.

Permeability of Molecules

• Permeability of molecules across the membrane is dependent on the charge type and size of the molecules.
• Some molecules can pass freely across membranes, others which need assistance to pass.
• Some molecules need assistance (e.g., membrane proteins) to pass over the membrane.

Facilitated Diffusion

• Facilitated transport relies on membrane protein binding sites and is saturable at high substrate concentrations.
• The rate of transport shows a hyperbolic relationship with substrate concentration, mirroring characteristics of enzymatic reactions.

Channel Proteins

• Membrane transporters facilitating diffusion also known as ion channel proteins.
• The structural integrity of membrane proteins is critical for function and involves several transmembrane segments.
• Important features of these proteins include selectivity, rapid ion conductance, and stimuli-dependent gating.

Potassium Ion Channel

• Potassium channels are essential for many cellular events, such as cell volume regulation, hormone release, and nerve impulse transmission.
• Each subunit contributes to potassium selectivity, due to five specific amino acids in the selectivity filter. (TVGYG)
• The backbone carbonyls and Thr hydroxyl residues bind to K⁺ ions; altering the sequence changes selectivity.

Gating the Potassium Channel

• The channel gate opening and closing response to specific stimuli, such as voltage and intracellular pH changes.
• Helix bending at glycine 99 acts as a molecular hinge, controlling channel opening/closing.

Active Transport

• Active transport moves substances against their concentration gradient, requiring energy input.
• Primary active transport uses ATP hydrolysis, light energy or electron flow to power transport.
• Secondary active transport utilizes the electrochemical gradient of one molecule to drive the movement of another.

Conformational Change in a Flippase

• A conformational change is a crucial component in the function of several transport proteins, including the flippase. This movement is necessary for many cellular functions.
• Examples were given to show how the movement is achieved.

Key Messages

• Conformational changes are key for transport, signaling, and regulating membrane protein activity.
• Non-steroid receptors are integral membrane proteins with varying structures and creating secondary messengers when activated/inactivated.
• Transport mechanisms can be concentration or ATP-dependent, depending on the protein structure and presence of co-transporters. These are general or specific.