Lecture 12: cell communicaion II

Question 1

Cell surface receptors:

1. G protein- coupled receptors

Answer 1

Associate with G proteins: bind GTP and GDP in the cytoplasm

G proteins bound to GDP: Inactive

G proteins bound to GTP: Active

Ligand binding to the receptor activates the G protein by replacing GDP with GTP

Question 2

Cell surface receptors:

1. G protein activation

Answer 2

• G proteins are composed of α, β, and γ subunits.
• α subunit: binds either GDP or GTP
• When α is bound to GDP: α, β, and γ associate and the G protein is
inactive
• When the G protein is bound by an
activated receptor: GDP is
replaced by GTP and the α subunit
separates from β and γ
• α-GTP binds activates target
proteins in the cell

Question 3

Adrenaline Signaling in Heart Muscle

Answer 3

Activated protein kinase A phosphorylates proteins in the heart muscle, causing heart rate to increase.

Question 4

Amplification of adrenaline signaling

Answer 4

• Activated receptors activate multiple G proteins. Which activates adenylyl cyclase enzymes
• adenylyl collapse enzymes produce cAMP. Activates protein kinase A.
• protein kinase A phosphorylates and activates proteins.

Question 5

Termination of adrenaline signaling.

Answer 5

• phosphatases remove phosphate groups from proteins. Makes them inactive.
• GTP to GDP
• cAMP to AMP

Question 6

Receptor kinases?

Answer 6

• Activated Receptor Kinases phosphorylate proteins in the cell

Question 7

The MAP Kinase Pathway and Wound Healing.

Answer 7

• PDGF: released by platelet cells in response to a wound
• PDGF activates PDGF receptor kinases at the site of the wound
• Activated receptors activate Ras (Ras-GDP à Ras-GTP)
• Ras-GTP triggers a series of kinase activations that reaches the nucleus
• Activated kinases in the nucleus phosphorylate proteins that regulate the transcription of genes required for cell division and wound healing

Question 8

Termination of RAS signaling.

Answer 8

• Phosphatases inactivate the receptor kinases and other kinases in the pathway

• Ras-GTP converts to Ras-GDP rendering it inactive; an active -
• Receptor Kinase is required to re- activate Ras
• Dissociation of PDGF from its receptor

Question 9

Ligand gated ion channels.

Answer 9

• Signaling molecule binds to a channel protein
• The channel undergoes a conformational change that opens
it and allows ions to flow into or out of the cell
• The channel remains open as long as the signaling molecule remains
bound

Question 10

Acetylcholine Receptors and Muscle Contractions

Answer 10

Acetylcholine binding opens ion channels allowing Na+ to flow into muscle cell.

Question 11

Cell signaling problems that chase cancer.

Answer 11

• Increased Receptors: increased binding to signaling molecules that can lead to abnormal gene express/excess cell division
• Overproduction/Altered signaling molecule: cancer cells can overproduce or make altered forms of a signaling molecule that can lead to increased binding to receptors and abnormal cell responses
• Ras Mutations: associated with 30% of all human cancers: Ras-GTP does not convert to Ras-GDP and is always active