Bio285 Topic 5

Cell signaling overarching principles

• Cells receive signals from their extracellular environment to sense things like amino acids, other cells, nutrients, ect, to communicate with other cells to figure out what is going on in the environment and to respond to it

• All organisms and cell types have the ability to receive signals internally and externally

• Cells can receive an array of signals at the same time including survival, growth and division, differentiation, and cell death

Signals and the phospholipid membrane

• Signals are typically small molecules or proteins, some signals are as small as nitric oxide or as large as an antibody

• Some signals can cross the cells plasma membrane and some cannot.

• If it cant it might be too hydrophilic to pass through the heads or too large.

• If it can it is small and hydrophobic enough to interact with tails but not repelled by heads

• If the signal cannot pass through the membrane there are protein receptors embedded within the cell membrane

• If the signal can pass through the membrane there will be receptors on the inside the cell that can bind the signal molecule

Two general outcomes of signal transduction

• Fast response by changing protein function

• Slow response when gene expression is changed

• Not mutually exclusive, both can happen as a result of a signal

Signal binding

• multiple cells can bind the same type of signal but yield a different outcome based on cell type

• The “message” of signaling molecule can be amplified during signal transduction by a molecule “turning on” multiple other molecules

Cross Talk

• Proteins in signal pathways can “talk” to each other by changing the function of proteins in other pathway

• Ex. a cell that is receiving a death signal to undergo programmed cell death would suppress other pathways for cell duplication

RTK/Ras/MapK pathway RTK activation

• Receptor tyrosine Kinase is the receptor (RTK).

• Signal binds to two inactive RTKs and bring them in close proximity. (called dimerization)

• Dimerization causes conformational shift in the kinase domains and activates the kinase activity which causes the two domains to phosphorylate each other

• The phosphorylation creates binding sites for signal transduction pathways by phosphorylating tyrosines

RTK//Ras/MapK pathway Grb2 and SOS

• The protein Grb2 binds to one of the phosphorylated tyrosines on RTK through the SH2 domain on Grb2

• SH2 domains are phosphotyrosine binding domains

• Grb2 is an adaptor protein to serve as a binding site for another protein called SOS which occurs through an SH3 domain on Grb2

• The SH3 domain binds to a PXXP motif on SOS

• SOS acts as a GEF and converts the GDP to GTP on Ras

RTK/Ras/MapK pathway Ras

• SOS acts as a Ras-GEF

• Ras is off in the GDP state and on in the GTP state

• Ras can hydrolyze GTP to GDP with help of a GAP

• When SOS binds to Ras it causes Ras to switch from a GDP bound state toa GTP bound state

• Ras:GTP then activates the MAP kinase cascade

RTK/Ras/MapK Map Kinase Cascade

• There are three kinase in the kinase cascade descending from MapK3 to MapK2 to MapK1

• MapK3 phosphorylates MapK2 to shift into active conformation which then allows it to bind ATP and then do hydrolysis

• Ras causes activation of MapK3

• MapK1 changes protein activity or changes gene expression

RTK/PI3 Kinase/AKT - PI3 Kinase

• End result is the inhibition of programmed cell death

• Survival signal binds an RTK

• RTK is activated by the same mechanism for RTK/Ras/MapK

• PI3 Kinase binds to phosphorylated RTK through its SH2 domain

• PI3 kinase phosphorylates a lipid rather than a protein.

• This forms PIP3 to serve as a binding site for downstream regulators

RTK/PI3 Kinase/AKT - AKT/PK1/PK2

• AKT is a downstream regulator, but AKT must be activated as a kinase

• AKT is brought to the inner surface by binding to PIP3 through a PH domain

• Another Kinase called Protein Kinase 1 also has a PH domain and is also recruited by PIP3

• PK1 phosphorylates AKT which causes AKT to shift conformation which causes AKT to release PIP3 and to begin diffusing in the cytosol

• The change exposes a second phosphorylate site for a second kinase

• The second kinase called PK2 (not attached to PIP3) phosphorylates AKT which activates it

RTK/PK3 Kinase/AKT - BAD/Bcl2

• BADs job when active (not phosphorylated) is to inhibit Bcl2

• Bcl2s job when not bound to BAD is to inhibit programmed cell death by binding to the mitochondrial membrane

• When BAD binds Bcl2 it signals for cell death

• The survival signaling pathway prevents cell death by activating AKT which phosphorylates and inactivates BAD and stops it from binding to Bcl2 and signaling for cell death

Jak/STAT pathway Jak1/Jak2

• Jak1 and Jak2 are non-receptor tyrosine kinases because they have tyrosine kinase domains that are similar to, but not RTKs

• Jak1 and Jak2 bind tightly to receptors to form a functional unit with quaternary structure

• Jak activation is very similar to RTK activation when dimerization brings Jak1 and Jak2 into close proximity to one another which activates their kinase activity.

• Jak1 and Jak2 phosphorylate each other and then phosphorylate the receptor they are bound to which creates binding sites for downstream proteins called STATs

Jak/STAT pathway - STAT

• STAT monomers each bind a phosphorylated receptor through the STATs SH2 domains

• Each STAT is in close proximity to its Jak

• Each Jak phosphorylates the STAT that is bound 

• Phosphorylation of STATs cause them to change shape, dissociate away from the receptor and bind to one another through their SH2 domains

• They are now active transcription factors and can be imported into the nucleus and activate transcription

GPCR/G protein signaling - GCPR

• GCPR works differently than RTK in that they don’t dimerize, they don’t have enzymatic activity, however, they change conformation upon signal binding

• When signal binds, GPCRs change shape around the signal which causes a shape change on the portion of the protein that is inside the cell

GPCR/G protein signaling - Ga

• The G protein complex makes interactions with the GCPR through the Ga subunit

• When GCPR changes conformation due to signal binding this causes a change in the conformation of Ga

• The change in conformation of Ga loses its affinity for GDP which causes it to dissociate and allows GTP to diffuse in

• GPCR acts as a GEF for Ga

• Once Ga is bound to GTP it changes conformation yet again and diffuses away from Gby and the GCPR

GPCR/G protein signaling - Gby and RGS

• When Ga dissociates away from Gby, this allows Gby to be active

• A GAP protein called RGS binds to Ga and helps Ga hydrolyze GTP

• When Ga hydrolyzes GTP to GDP it shifts into the off conformation

• Ga then diffuses back to Gby shutting them down