Second Messenger, Catalytic, and Steroid Signaling Flashcards

Vocabulary flashcards covering cellular signaling concepts, including G-protein-coupled receptors, catalytic tyrosine kinase pathways, second messenger cascades, and steroid hormone signaling mechanisms.

Target Cell

A cell that receives a chemical signaling message by expressing specific protein receptors capable of binding a signaling molecule.

G Protein-Linked Receptors

Transmembrane receptor proteins containing seven membrane-spanning helices that activate heterotrimeric G proteins to regulate second messenger production and amplify signals.

Heterotrimeric G Proteins

Membrane-associated signaling complexes composed of three distinct subunits ($\text{α}$, $\text{β}$, and $\text{γ}$) that bind $GTP$ when active and $GDP$ when inactive.

Ras Superfamily G Proteins

Monomeric small G proteins homologous to the $\text{α}$ subunit of heterotrimeric G proteins that act as molecular switches for cell growth and differentiation downstream of catalytic receptors.

Adenylyl Cyclase

A membrane-bound effector enzyme activated by stimulated $G_s$ that converts $ATP$ into the second messenger cyclic $AMP$ ($cAMP$).

Cyclic AMP ($cAMP$)

A key second messenger produced by adenylyl cyclase that activates Protein Kinase A ($PKA$) to relay and amplify intracellular signaling cascades.

Protein Kinase A ($PKA$)

A $cAMP$-regulated serine/threonine kinase that transfers phosphate groups from $ATP$ to serine and threonine residues on specific protein substrates.

Cholera Toxin

A bacterial toxin that modifies the $\text{α}$ subunit of $G_s$ in intestinal epithelial cells, blocking $GTP$ cleavage and causing persistent overproduction of $cAMP$, severe diarrhea, and dehydration.

Pertussis Toxin

A bacterial toxin that disrupts $G_i$ activity in the respiratory tract by inhibiting its $\text{α}$ subunit, leaving adenylyl cyclase continuously active to generate excess $cAMP$.

Phospholipase C ($PLC$)

A membrane enzyme activated by $G_q$ or $G_p$ proteins that cleaves phosphatidylinositol 4,5-bisphosphate ($PIP_2$) into the second messengers $IP_3$ and $DAG$.

Inositol 1,4,5-Trisphosphate ($IP_3$)

A soluble second messenger produced by $PLC$ cleavage of $PIP_2$ that binds to specific receptors on the endoplasmic reticulum to trigger the release of stored $Ca^{2+}$.

Diacylglycerol ($DAG$)

A membrane-anchored second messenger generated by $PLC$ cleavage of $PIP_2$ that works together with $Ca^{2+}$ to activate Protein Kinase C ($PKC$).

Protein Kinase C ($PKC$)

A serine/threonine kinase activated by diacylglycerol ($DAG$) and $Ca^{2+}$ that phosphorylates cellular proteins involved in cell proliferation, differentiation, and other responses.

Calmodulin

An intracellular calcium-binding protein that forms a $Ca^{2+}$-calmodulin complex during transient rises in cytosolic $Ca^{2+}$, activating various $Ca^{2+}$-dependent enzymes.

Catalytic Receptors

Transmembrane receptors that signal by phosphorylating tyrosine residues on intracellular substrate proteins, operating either through intrinsic tyrosine kinase domains or associated non-receptor tyrosine kinases.

Adaptor Proteins

Intracellular signaling proteins containing conserved domains (such as $SH2$ and $SH3$) that dock at phosphotyrosine residues on receptor tails to assemble intracellular signaling complexes.

SH2 Domain

A highly conserved protein domain (Src homology 2) that specifically recognizes and binds to phosphotyrosine residues and surrounding amino acids on target proteins.

MAPK Cascade

A sequential serine/threonine phosphorylation pathway ($MAPKKK → MEK → MAPK/ERK$) activated downstream of Ras that translocates to the nucleus to phosphorylate transcription factors.

STATs

Signal transducers and activators of transcription; latent cytoplasmic adaptor proteins with $SH2$ domains that become tyrosine-phosphorylated by receptor kinases, dimerize, and translocate to the nucleus to activate gene transcription.

PI3 Kinase Pathway

A major cell survival signaling pathway in which $PI_3$ kinase binds to phosphotyrosine residues and converts membrane $PIP_2$ into $PIP_3$, recruiting and activating Akt.

PTEN

A phosphatase that dephosphorylates $PIP_3$ back to $PIP_2$, effectively inactivating the $PI_3$ kinase signaling cascade.

Insulin Receptor Substrates ($IRS$)

Intracellular proteins phosphorylated on tyrosine residues by the activated insulin receptor kinase, serving as docking sites for $SH2$-containing signaling molecules like STATs, $PI_3$ kinase, and Ras adaptors.

Nuclear-Initiated Steroid Signaling ($NISS$)

The classical, slower steroid signaling pathway where lipophilic steroid hormones cross the cell membrane, bind intracellular receptors in the cytosol or nucleus, and directly direct gene transcription.

Membrane-Initiated Steroid Signaling ($MISS$)

A rapid steroid signaling mechanism mediated by steroid receptors localized in membrane caveolae that trigger protein kinase cascades without requiring new protein synthesis.