Exam 4: Lecture 4

Question 1

Signal Tranduction Cascade

Answer 1

• typically will involve the production of ligand by sending cell
• this molecule will bind to cell surface transmembrane receptor on receiving cell
• activation of receptor will lead to transmission of instructions to nucleus through serial activation of several cytoplasmic factors
• most downstream member of cascade translocates into nucleus where it will physically modify transcription factors (usually via phosphorylation)

Question 2

Phosphate Groups

Answer 2

• addition to TFs can sometimes lead to inactivation of protein
• or can lead to activation

Question 3

Morphogens

Answer 3

-proteins secreted at one cell to act at a distance to influence the cells that it surrounds.

Question 4

Paracrine Signaling

Answer 4

• used by long range signaling gradient
• diffuse past most immediate cell neighbors and will make contact with receptors located further away from sending cell
• some signaling molecules are first generated as larger proteins that can be tethered to surface of sending cell plasma membrane
• in response to cleavage event, biologically active ligand can diffuse away-also signal at both short and long range distances

Question 5

Morphogen Gradient

Answer 5

• ligand which is produced and secreted from small population of cells is capable of travelling several cell diameters
• Each receiving cell will express the same number of receptor molecules
• however cell closest to sending cell will have highest number of occupied receptors
• more distant cells have fewer occupied receptors
• gradation in receptor-ligand binding can lead to differential levels of activated transcription factors which influences gene expression

Question 6

Gradient Levels and Transcription

Answer 6

• cells that are closest have highest levels of ligand-receptor bound complexes.
• leads to highest level of activated TF and in turn all 3 target genes (A, B, and C) actively transcribed
• intermediate cells have fewer occupied receptors and therefore less activated TFs and only genes B and C transcribed
• far cells have least number of occupied receptors and activated TFs so only gene C is transcribed.
• picture on slide

Question 7

Spatzle

Answer 7

• secreted by ventral follicle cells that surround developing Drosophila embryo.
• capture of this ligand by Toll receptor leads to translocation of Dorsal TF into nucleus
• vast majority captured by receptors on ventral cells
• remaining captured by Toll receptors on lateral cells
• dorsal receptors never receive

Question 8

Juxtacrine Signaling

Answer 8

• tethered to membrane and are never cleaved or secreted by sending cell
• can only signal to cells directly contacting sending cell
• some cases ligand-receptor complex will be internalized within receiving cell
• other cases simple binding of tethered ligand to transmembrane receptor is sufficient to activate receptor and initiate signal transduction

Question 9

Autocrine Signaling

Answer 9

• situations in which diffusible ligand is secreted by ell then binds to cell surface receptors on same side of cell
• leads to biochemical changes within same cells
• cells that respond to its environment and releases proteins into cellular membrane, but only affects itself

Question 10

Intracrine Signaling

Answer 10

• ligand or hormone enters cell and goes straight to nucleus without interacting with receptors at cell surface
• once in nucleus, bind to nuclear receptors
• together steroid-receptor complex will bind to DNA and activate/repress transcription of target genes

Question 11

Endocrine

Answer 11

-ligand/morphogen is secreted into circulatory system to be carried to distant organ or cell population

Question 12

Determining Ligand Diffusion Distance

Answer 12

-see example on slide

Question 13

Mechanism of Receptor Tyrosine Kinase Activity (RTK)

Answer 13

• mediates many signal transduction cascades
• prior to signal activation, ligand and receptors not bound to each other
• absence of ligand, receptors exist as monomers within plasma membrane
• binding of ligands and receptors changes conformation of receptor so they can physically interact with each other within plasma membrane
• RTKs form dimer complexes
• dimerization triggers activation of kinase domains located in cytoplasmic sections of receptors
• kinase domain is responsible for adding phosphate groups
• RTKs trans phosphorylate each other at conserved tyrosine amino acids located within cytoplasmic sections of protein
• phosphorylated receptor now active and will stimulate activity of cytoplasmic proteins

Question 14

Mechanism of Receptor Tyrosine Kinase Activity (Cytoplasmic Signaling Cascade)

Answer 14

• binding of ligand to RTK leads to dimerization and transphosphorylation of receptor
• leads to activation of cytoplasmic signaling cascade
• first molecule activated is Ras which is a GTPase
• Ras activates cytoplasmic Raf kinase
• Raf kinase phosphorylates and activates MEK
• MEK phosphorylates and activates MAPK
• unphosphorylated MAPK found in cytoplasm and dually phosphorylated version is translocated into nucleus
• after MAPK enters nucleus it will phosphorylate several TFs
• some activators will then promote gene expression
• other receptors inhibit gene expression.

Question 15

RTKs

Answer 15

• all RTKs transmit signals to nucleus through Ras/MAPK signaling cascade
• activation of different RTKs will lead to modification of very different transcription factors
• different RTKs can function in different tissues while multiple RTKs can also function within same cell

Question 16

Toll Receptor

Answer 16

• unbound toll receptors bound by Tube and Pelle proteins
• when Spatzle ligand binds to Toll receptor the two proteins are released from receptor and quickly bind to Cactus
• before Toll receptor activation, Cactus binds and sequesters Dorsal TF in cytoplasm
• when Tube and Pelle released from Toll receptor they bind and participate in degradation of Cactus
• allows Dorsal protein to enter nucleus and activate transcription of target genes (twist, rhomboid, and sog)

Question 17

Notch Pathway

Answer 17

• Delta ligand binds to Notch receptor and Notch receptor is then bound by cytoplasmic scissor protein Kuz
• Kuz cleaves intracellular domain of receptor Nicd
• Nicd then translocated to nucleus where it interacts with Su(H) DNA BP
• absence of Notch signaling Su(H) physically interacts with C terminal BP (CtBP) and Groucho (Gro) co-repressors to inhibit transcription
• Nicd displaces both of these which turns Su(H) into transcriptional activator
• cells that express Notch receptor become non-neuronal cells while those that express Delta become neurons

Question 18

Hedgehog Pathway

Answer 18

• absence of Hh ligand and Patched (Ptc) receptor will bind to and inhibit activity of another membrane receptor called Smoothened (Smo)
• within cytoplasm dozen proteins bound together as complex whose role is to partial degrade Tf called Cubitus Interruptus (Ci)
• smaller factor enters nucleus and functions as transcriptional repressor
• Hh ligand binds to Ptc receptor preventing it from binding to Smo receptor
• consequently they cytoplasmic protein complex dissociates and full-length Ci protein is allowed to enter nucleus.
• full-length Ci is activator