Second Messenger, Catalytic, and Steroid Signaling

Flashcards covering G-protein signaling, adenylyl cyclase, phospholipase C, catalytic receptors, MAP kinase, STAT, PI3K pathways, insulin signaling, and nuclear vs. membrane steroid signaling.

What is a target cell in cell signaling?

A target cell is a cell that receives a chemical signaling message (such as a hormone, growth factor, or neurotransmitter) via specific protein receptors.

What are the three primary mechanisms/types of cell signaling receptors described in the lecture?

G protein-linked receptors (which produce second messengers), catalytic receptors (which phosphorylate tyrosine residues), and intracellular receptors (which bind hydrophobic molecules like steroid hormones).

What is the structural architecture of heterotrimeric G-protein-coupled receptors (GPCRs)?

They are transmembrane proteins with seven membrane-spanning helices ($7\text{TM}$), an extracellular domain containing the ligand-binding site, and an intracellular domain that interacts with G proteins.

What are the structural and functional differences between heterotrimeric G proteins and Ras superfamily G proteins as shown in this diagram?

Heterotrimeric G proteins consist of three subunits ($\alpha, \beta, \gamma$), interact with G-protein-linked receptors, and regulate second messengers. Ras superfamily G proteins are monomers resembling the $\alpha$ subunit and interact with catalytic receptors.

What three subunits compose a heterotrimeric G protein, and what is bound to the protein in its inactive state?

It consists of $\alpha$, $\beta$, and $\gamma$ subunits and is bound to guanosine diphosphate ($\text{GDP}$) when inactive.

Approximately how many distinct G-proteins and G-protein-coupled receptors have been identified in humans?

Approximately $400$ distinct G-proteins and $900$ distinct G-protein-coupled receptors.

What molecular activation steps occur immediately following ligand binding to a GPCR?

The receptor undergoes a conformational change and interacts with the G protein; the $\alpha$ subunit releases $\text{GDP}$ and binds $\text{GTP}$, then dissociates from the $\beta$ and $\gamma$ subunits to interact with a membrane enzyme.

How is heterotrimeric G-protein signaling turned off by the cell?

The $\alpha$ subunit hydrolyzes bound $\text{GTP}$ to $\text{GDP}$ through its intrinsic GTPase activity, inactivating the G protein and allowing the $\alpha$ subunit to reassociate with $\beta$ and $\gamma$ subunits.

Which enzyme is stimulated by active $G_s$, and what second messenger does it produce?

Active $G_s$ stimulates adenylyl cyclase, which converts $\text{ATP}$ into cyclic adenosine monophosphate ($\text{cAMP}$) and inorganic pyrophosphate ($\text{PP}_i$).

How do $G_s$ and $G_i$ proteins differ in their regulation of adenylyl cyclase?

$G_s$ stimulates adenylyl cyclase activity to increase $\text{cAMP}$ production, whereas $G_i$ inhibits adenylyl cyclase activity to reduce $\text{cAMP}$ production.

Which kinase is activated by $\text{cAMP}$, and what residues does it phosphorylate?

$\text{cAMP}$ activates protein kinase A ($\text{PKA}$), which transfers phosphate groups from $\text{ATP}$ to serine (Ser) and threonine (Thr) residues on target proteins.

What is the mechanism of action of cholera toxin in intestinal epithelial cells?

Cholera toxin modifies the $\alpha$ subunit of $G_s$, inhibiting its ability to cleave $\text{GTP}$ to $\text{GDP}$. This causes overproduction of $\text{cAMP}$, leading to continuous transport of sodium and water into the gut lumen, causing severe diarrhea and dehydration.

What is the mechanism of action of pertussis toxin in respiratory tract cells?

Pertussis toxin inhibits the $\alpha$ subunit of $G_i$, preventing adenylyl cyclase from being turned off. Adenylyl cyclase remains active indefinitely, producing excess $\text{cAMP}$ and leading to coughing, vomiting, and dehydration.

Which G proteins activate Phospholipase C (PLC), and what products are generated by PLC cleavage?

$G_q$ and $G_p$ activate Phospholipase C, which cleaves phosphatidylinositol 4,5-bisphosphate ($\text{PIP}_2$) into inositol 1,4,5-trisphosphate ($\text{IP}_3$) and diacylglycerol ($\text{DAG}$).

What are the distinct fates and actions of $\text{IP}_3$ and $\text{DAG}$ after Phospholipase C cleavage?

$\text{IP}_3$ diffuses to the endoplasmic reticulum to release stored $\text{Ca}^{2+}$ into the cytosol, while $\text{DAG}$ remains anchored in the plasma membrane and activates Protein Kinase C ($\text{PKC}$) along with $\text{Ca}^{2+}$.

What happens when cytosolic $\text{Ca}^{2+}$ concentration increases following $\text{IP}_3$ signaling?

$\text{Ca}^{2+}$ binds to calmodulin, forming the $\text{Ca}^{2+}$-calmodulin complex that binds to and activates many $\text{Ca}^{2+}$-dependent cellular enzymes.

What three domains compose single-chain catalytic receptors with intrinsic tyrosine kinase activity?

An extracellular $\text{NH}_2$-terminal ligand-binding portion, a single $\alpha$-helical membrane-spanning domain, and an intracellular effector domain containing the tyrosine kinase function.

How does ligand binding activate catalytic receptors with intrinsic tyrosine kinase activity?

Ligand binding brings two or more receptor chains together to form dimers; the close proximity of the intracellular tyrosine kinase domains allows them to cross-phosphorylate (autophosphorylate) tyrosine residues on opposite tails.

What function do $\text{SH2}$ domains serve in catalytic receptor signaling?

$\text{SH2}$ (Src homology 2) domains on adaptor proteins specifically dock onto phosphorylated tyrosine residues and neighboring amino acid sequences on activated receptor cytoplasmic tails.

What is Ras, and what is its significance in human cancers?

Ras is a monomeric GTP-binding protein (small G protein) that controls cell growth and differentiation; approximately $30\%$ of human tumors carry constitutively active mutant forms of Ras.

What is the sequence of kinases activated in the MAP kinase cascade downstream of Ras-GTP?

Active Ras-GTP activates RAF ($\text{MAPKKK}$), RAF phosphorylates MEK ($\text{MAPKK}$), MEK phosphorylates $\text{MAPK}$ ($\text{ERK}$), and $\text{MAPK}$ translocates to the nucleus to phosphorylate transcription factors.

How do STAT proteins relay signals from catalytic receptors to the nucleus?

$\text{SH2}$-domain-containing STATs dock at phosphotyrosines on receptor tails, are phosphorylated on tyrosines by the receptor kinase, form dimers, translocate to the nucleus, and activate gene transcription.

What is the function of $\text{PI3}$ kinase and PTEN in the $\text{PI3}$ kinase pathway?

$\text{PI3}$ kinase phosphorylates $\text{PIP}_2$ to generate $\text{PIP}_3$, which recruits and activates Akt to promote cell survival. PTEN dephosphorylates $\text{PIP}_3$ back to $\text{PIP}_2$ to halt signaling.

How do cytokine receptors signal if they lack intrinsic tyrosine kinase activity?

They non-covalently associate with cytoplasmic non-receptor tyrosine kinases (such as Src or Janus kinase/JAK family members), which phosphorylate tyrosine residues on the receptor tail upon ligand binding.

What is the structure of the insulin receptor prior to ligand binding?

It is a pre-formed heterotetramer composed of two extracellular $\alpha$ subunits and two transmembrane $\beta$ subunits linked together by disulfide ($\text{S-S}$) bonds.

What major biological metabolic effects are promoted by insulin signaling?

Insulin increases glucose uptake, glycogen synthesis, protein synthesis, and fat synthesis, while decreasing gluconeogenesis.

How does insulin facilitate glucose uptake in skeletal muscle and adipose tissue?

Insulin signaling promotes recruitment and fusion of intracellular vesicles containing GLUT4 glucose transporters with the plasma membrane, allowing glucose entry into the cell.

What are the structural domains of an intracellular steroid hormone receptor as depicted in this diagram?

An $\text{NH}_2$-terminal gene regulatory domain, a DNA-binding domain, a hinge region, and a COOH-terminal hormone-binding domain with a hormone binding site.

How does cortisol activate gene transcription via nuclear-initiated steroid signaling (NISS)?

Cortisol crosses the plasma membrane, binds the glucocorticoid receptor (GR) to uncover its zinc finger DNA-binding domain, dimerizes, translocates to the nucleus, and binds Glucocorticoid Response Elements (GRE) in enhancer regions to activate transcription.

How does membrane-initiated steroid signaling (MISS) differ from nuclear-initiated steroid signaling (NISS)?

MISS acts rapidly (seconds to minutes) through membrane-bound steroid receptors in caveolae that associate with G proteins and kinases to modify existing proteins, whereas NISS acts slowly (hours) by altering gene transcription and protein synthesis.