Molecular and Cell Biology - Cell Signaling Pt.2

Define signal transduction

• When a ligand binds to a receptor on the cell surface, which initiates intracellular reactions

What are G-proteins?

• They are the largest family of cell surface receptors derived from the guanine nucleotide-binding proteins

  • G-proteins are the receptors for many hormones and neurotransmitters

  • Responsible for smell, sight, and taste

Describe the important components of G-protein-coupled receptors

• Carbohydrates are required for proper folding of receptor

• G protein coupled receptors have seven transmembrane alpha helices, spanning across the cell membrane

• Intracellular ligand binding domain - has guanine nucleotide exchange factor (GEF) activity

• An extracellular ligand binding domain

Describe the activation of adenylyl cyclase by epinephrine

1. Hormone (epinephrine) binds to extracellular domain of the receptor

2. Binding of the hormone cause a conformational change in the intra-cellular domain, which activates the G protein associated with it

3. G protein dissociates from the receptor and carriers the signal to the intracellular target (enzymes, ion-channels)

What is the function of epinephrine?

• Epinephrine regulates the synthesis of cyclic AMP, cAMP is an important secondary messenger that mediates a number of cellular responses

Understand the diagram showing the GPCR signaling

What do G-proteins look like when they are inactivated

• They are inactivated when there is an absence of the hormone

  • In this state the a subunit is bound to GDP in complex with B and y

Describe the G-proteins when the hormone binds

• Hormone binds to the extracellular domain of the G protein-coupled receptor

• The activated G Protein couple acts as a guanine nucleotide exchange factor

• Binding of the hormone stimulates the release of GDP

• GDP is exchange for GTP

Describe G-proteins that are activated

• GTP is bound to the a subunit activating the G protein

• The activated GTP-bound a subunit and By complex dissociate from the receptor

• The a subunit and By complex interacts with their targets

What are G-proteins regulated by?

GTPase-activating protein (GAP)

• GTP hydrolysis is facilitated by GAP

• Without GAP, G proteins are constitutively on because of their slow hydrolytic activity and GEFs constantly replacing GDP. This results in unregulated cell division

What signaling molecule signals the “fight or flight” response

• Epinephrine

Why does the epinephrine signaling pathway have so many steps in the liver?

• Allows for cross talk

• Allows for amplification

Describe the specificity of cell signaling

• The final effect is mostly determined by the signaling pathway that is activated by the receptor

• The same signal can activate different responses in different cells

How are G proteins divided into their four families

• G proteins are divided into four families based on their a subunit

  • Each family triggers a different signaling pathway

• The effect a ligand will have in a cell depends on the specific pathway that is triggered

What are G_i proteins?

G_i proteins have a subunits that inhibit target enzymes (Adenylyl cyclase

Describe the G-protein in the eye

• It is called Rhodopsin

• Light (signal) changes the conformation of retinal from cis to trans

• This causes activation of the receptor and exchange fro, GDP to GTP in the G protein (transducing)

  • Phosphodiesterase turns cGMP into GMP

  • Lack of cGMP will have an effect in the cell

Understand the pathway of G protein epinenephrine activation

Describe the process of how G-proteins pass information into the nucleus

1. Signals leads to activation of the catalytic domain of Protein Kinase A

2. Signal translocates from the cytoplasm to the nucleus

3. PKA catalytic subunit then phosphorylates a transcription factor called CRE-binding protein (CREB)

4. Co-factor binds CREB

5. Transcription of cAMP-inducible genes

Define what kinasese do and what Phosphatases

• Kinases: phosphorylate

• Phosphatases: de-phosphorylate

What are the two classes of kinases/phosphatases?

• Serine/theronine kinase

• Tyrosine kinase

What is the result of phosphorylation?

• Can change the shape of the protein (usually means activation)

• Can create a binding site for other proteins (mostly true for tyrosine)

What are the two functions of Receptor Tyrosine Kinases (RTKs)?

• Bind to a signal

• Function as a kinase

What form do RTKs take? Describe the process of how they change forms

• Some can be in a dimeric form, but the majority dimerize after ligand bonding

• Ligand bonding causes dimerization which brings the kinase domains together

  • Leads to cross-phosphorylation (the kinase domains will phosphorylate each other)

• Phosphorylation of the receptors creates binding sites for other proteins (specifically those that have SH2 domain)

Describe Non-Receptor Tyrosine Kinases

• The receptor only binds to the ligand

  • does not have kinase activity

• The receptor does that a protein assocaited with it that functions as a kinase

  • The receptor and kinase are different proteins

• This allow different proteins to be matched together

What does ligand bonding lead to in Non-Receptor Tyrosine Kinases?

• Ligand bind causes dimerization which brings the kinases together

• Leads to cross-phosphorylation AND phosphorylation of the receptor

  • This creates binding sites for proteins that have SH2 domains

What is the JAK/STAT Pathway activated by? Define what JAK and STAT are in this pathway

• This pathway can be activated by cytokines

  • cytokines are peptides that allow immune cell to communicate)

• JAK (the kinase) phosphorylates the receptor, creating binding sites

  • A non-receptor tyrosine kinase

• STAT has an SH2 domain and binds to the receptor where it will be phosphorylated

  • Singal transducer and activator of transcription

Describe what ahppens once STATs are phosphorylated. What is the end result of this pathway?

• Phosphorylated STATs dimerize and move to the nucleus to act as a transcription factor

• Cells release cytokines to tell other cells to produce specific proteins

What are integrins? What is a non-receptor tyrosine kinase that is a integrin?

• Integrins are transmembrane proteins that interact with the extracellular matrix

  • Play a major role in migration, poliferation, and cell survival

• FAK is the n-RTK

  • FAKs interaction with the extracellular matrix causes the integrins to cluster leading to cross-phosphorylation

What are MAP kinase pathways?

• They are a major pathway activated by enzyme-linked receptors

• Refer to a cascade of protein kinases that is highly conserved in eukaryotes

  • They are serine/threonine kinases

Ras is a growth factor associated with MAP kinase pathways, describe Ras and its function

• Ras is a lipid-anchored protein that with the help of a GEF can become activated

  • GEF is a guanine nucleotide exchange factor

How do the different proteins of the MAP pathway find each other inside the cell?

• Scaffold proteins bring different component of the signaling pathway in close proximity

  • This aids in the regulation of this pathway. If left unregulated can cause cancer

Describe how PI3-kinase/Akt pathway is activated and what is its function

• Usually activated by a receptor tyrosine kinases

  • Can also be activated via G-protein coupled receptors

• Important for cell proliferation and survival