Signal Transduction in Biology

Signal Transduction in Biological Membranes

• Signal transduction involves cells responding to extracellular molecules (e.g., hormones, neurotransmitters, growth factors) through specific receptors.
• Receptors can be intracellular (e.g., steroid and thyroid hormones) or extracellular (located on the plasma membrane).
• Most signaling molecules don't readily cross the plasma membrane, hence more receptors are on the cell surface.

Objectives

• Understand how cells recognize signaling molecules and trigger appropriate cellular responses.
• Understand how activated G protein-coupled receptors initiate a chain of signaling events involving G proteins and effector proteins (enzymes or ion channels).
• The objective is further to understand how cells recognize signaling molecules to evoke a suitable cellular effect.

What is Signal Transduction?

• Extracellular signaling molecules (e.g., hormones) require receptors to trigger responses in cells.
• Receptors may be intracellular (e.g., for steroid hormones) or extracellular (e.g., on the plasma membrane), depending on the signal molecule.
• The majority of signaling molecules don't readily cross the plasma membrane; therefore, more receptors are located at the cell surface.

Typical Signal Transduction Pathway

• Signal molecules from the extracellular fluid bind to specific receptors on the plasma membrane.
• The signal binding and receptor activation initiates a signaling pathway within the cell.
• The pathway can trigger cellular responses like contraction, secretion, proliferation, and differentiation.

G Protein-Coupled Receptors (GPCRs)

• GPCRs are a large family of cell surface receptors.
• GPCRs alter the activity of effectors (enzymes, ions, or channels).
• They achieve this via the activation of guanine nucleotide binding proteins (G proteins).
• The GPCRs' interaction with the GPCR initiates a cascade of cellular responses.

GPCR Features

• Single polypeptide chain (300-1200 amino acids).
• 7 transmembrane (7TM) spanning regions.
• Extracellular N-terminal and intracellular C-terminal.
• GPCRs number more than 700 in the human genome.

How GPCRs Cause Cellular Activity Changes

• Agonist binds to the receptor.
• Protein-protein interactions release GDP and bind GTP.
• Released GTP interacts with effectors.
• GTP hydrolyzes to GDP.
• GDP reforms the heterotrimeric G protein.

G Protein Subunits and Their Function

• G proteins are heterotrimeric, composed of α, β, and γ subunits.
• Functional units include α and βγ subunits.
• GPCR interaction activates the G protein by GTP exchange for GDP on the G protein a subunit.

α-βγ Complex Dissociation

• Dissociated α-βγ complex interacts with effector proteins.

• Effectors may be second messenger-generating enzymes or ion channels.

• α subunit's activity eventually ceases after GTP hydrolysis back to GDP.

• α-GDP and βγ subunits reform the inactive heterotrimeric complex.

G Protein Diversity

• The human genome encodes multiple Gα, Gβ, and Gγ proteins.
• This creates numerous possible Gα-βγ protein combinations.
• Specific GPCRs preferentially interact with particular G protein types.
• Extracellular signal via a specific GPCR activates a particular G protein to bring about a specific cellular response.

G Protein GTP/GDP Exchange and GTP Hydrolysis

• The G protein acts as an on/off switch and timer.
• GTP exchange and hydrolysis govern the duration of signal transduction effects.
• The length of time taken for GTP hydrolysis is an essential aspect of signal termination.

Cellular Targets for Activated G Proteins

• Activated G proteins stimulate specific intracellular targets(e.g., enzymes).
• Some activated G proteins trigger specific intracellular metabolic pathways, for instance, regulating cAMP levels(adenylyl cyclase).
• Other activated G proteins interact with phospholipase C to produce inositol and diacylglycerol.
• Light-sensitive rhodopsin activates a G protein-mediated pathway.

GPCR Inhibition and Regulation

• Some G proteins are inhibitory (Gi) and decrease cAMP levels by inhibiting adenylyl cyclase.

• Inhibitory G proteins (Gq/11) interact with membrane-bound phospholipase C, generating inositol and diacylglycerol second messengers.

• Photoreceptor G protein (transducing Gprotein) activates a cascade that eventually leads to hydrolyzing cyclic GMP to 5'-GMP.

Experimental Manipulation of the G Protein Cycle

• Cholera toxin (CTx) eliminates G protein GTPase activity, leading to irreversible activation.
• Pertussis toxin (PTx) interferes with G protein GDP/GTP exchange leading to irreversible inactivation.
• Both CTx and PTx utilize ADP-ribosyl transferase for covalent modifications of specific G proteins.

Diseases Associated with Signal Transduction in GPCRs

• Genetic changes in GPCRs can cause loss-of-function or gain-of-function mutations.
• Examples include retinitis pigmentosa (loss-of-function in rhodopsin), nephrogenic diabetes insipidus (loss-of-function in vasopressin receptor), and familial male precocious puberty (gain-of-function in LH receptor).