1. Signal Transduction Pathways
“From signal to gene expression”

2. Signal Transduction Pathways
The biochemical events that conduct the signal of a hormone or growth factor from the cell exterior, through the cell membrane and into the cytoplasm which involve a number of molecules including receptors, proteins, and messengers
a series of steps by which a signal on a cell’s surface is converted into a specific cellular response

3. Overview on Plant Signal Transduction
The stream of signals to which plant cells react is continuous and complex
Signal transduction uses a network of interactions within cells, and throughout the plants

4. cell signaling transduction pathway
Analogy view of
cell signaling transduction pathway
Input
(command)
Keyboard
Signal
perception
Endogenous:
phytohormone
Exogenous:
environmental cue
CPU
(Central Processing
Unit)
Chip
Signal Transduction
Network
Compiling,
Integration,
processing
Output
(performance)
Printer
Signal
Response
Morphogenesis change,
Growth development
differentiation

5. EXTRACELLULAR
FLUID
Reception
Transduction
CYTOPLASM
Plasma membrane
Receptor
Signal
molecule

6. Relay molecules in a signal transduction
EXTRACELLULAR
FLUID
CYTOPLASM
Reception
Transduction
Plasma membrane
Receptor
Relay molecules in a signal transduction
pathway
Signal
molecule

7. Relay molecules in a signal transduction
EXTRACELLULAR
FLUID
Reception
CYTOPLASM
Transduction
Response
Plasma membrane
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction
pathway
Signal
molecule

8. Overview on Plant Signal Transduction
The stream of signals to which plant cells react is continuous and complex
Signal transduction uses a network of interactions within cells, and throughout the plants

10. Signal transduction (Simplified model) STIMULUS G-prot R Ca2+ Ca2+ R
Plasma
membrane R
G-prot
Ca2+
Phos
Ca2+
Kin
Nuclear
membrane R TF
DNA

11. Signal Transduction Components
Stimulus
Hormones, physical environment, pathogens
Receptor
On the plasmamembrane, or internal
Secondary messengers
Ca2+, G-proteins, Inositol Phosphate
Effector molecules
Protein kinases or phosphatases
Transcription factors
Response
Stomatal closure
Change in growth direction

12. STIMULUS How many signal factors which Plant will respond to?
--biotic and abiotic factors--

13. Stimulus

14. Reception
A signal molecule binds to a receptor protein, causing it to change shape
The binding between a signal molecule (ligand) and receptor is highly specific
A conformational change in a receptor is often the initial transduction of the signal
Most signal receptors are plasma membrane proteins

15. Intracellular Receptors
Some intracellular receptor proteins are found in the cytosol or nucleus of target cells
Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors
An activated hormone-receptor complex can act as a transcription factor, turning on specific genes

16. Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane
There are three main types of membrane receptors:
1. G-protein-linked receptors
2. Receptor tyrosine kinases
3. Ion channel receptors

17. G-protein-linked Receptor
It is a plasma membrane receptor that works with the help of a G protein
The G-protein acts as an on/off switch
If GDP is bound to the G protein, the G protein is inactive

18. G-protein-linked receptor
Signal-binding site
Segment that
interacts with
G proteins
G-protein-linked receptor

19. Receptor Tyrosine Kinases
Regulates cell growth and cell reproduction
Tyrosine kinase catalyzes transfer of phosphate group from ATP to tyrosines
Can trigger ten or more signal transduction pathways at once
Abnormal tyrosine kinases that work even without a signal molecule may contribute to some cancers

20. Fully activated receptor tyrosine-kinase (phosphorylated dimer)
Signal
molecule
Signal-binding site
a Helix in the
membrane
Signal
molecule
Tyrosines
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine
kinase proteins
(inactive monomers)
Dimer
CYTOPLASM
Activated relay
proteins
Cellular
response 1
Tyr
Tyr P
Tyr P
Tyr P
Tyr
Tyr P
Tyr
Tyr P
Tyr
Tyr P P P
Tyr
Tyr
Tyr
Tyr P
Tyr
Tyr P P
Tyr
Tyr P
Cellular
response 2 6
ATP
6 ADP
Activated tyrosine-
kinase regions
(unphosphorylated
dimer)
Fully activated receptor
tyrosine-kinase
(phosphorylated
dimer)
Inactive
relay proteins

21. Ion Channel Receptor Acts as a gate when the receptor changes shape
Signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor

22. Receptor

23. Receptor

24. Receptor

25. Receptor

26. Receptor

27. Receptor

29. Transduction Transduction usually involves multiple steps
Cascades of molecular interactions relay signals from receptors to target molecules in the cell
Transduction usually involves multiple steps
Multistep pathways can amplify a signal:
A few molecules can produce a large cellular response
Multistep pathways provide more opportunities for coordination and regulation

30. Signal Transduction Pathways
The molecules that relay a signal from receptor to response are mostly proteins
Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated
At each step, the signal is transduced into a different form, usually a conformational change

31. Protein Phosphorylation and Dephosphorylation
In many pathways, the signal is transmitted by a cascade of protein phosphorylations
Phosphatase enzymes remove the phosphates
This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off

32. Phosphorylation cascade
Signal molecule
Receptor
Activated relay
molecule
Inactive
protein kinase 1
Active
protein
kinase 1
Inactive
protein kinase 2
ATP
ADP
Active
protein
kinase 2 P
Phosphorylation cascade PP P i
Inactive
protein kinase 3
ATP
ADP
Active
protein
kinase 3 P PP P i
Inactive
protein
ATP
ADP P
Active
protein
Cellular
response PP P i

33. Small Molecules and Ions as Second Messengers
Second messengers are small, nonprotein, water-soluble molecules or ions
The extracellular signal molecule that binds to the membrane is a pathway’s “first messenger”
Second messengers can readily spread throughout cells by diffusion
Second messengers participate in pathways initiated by G-protein-linked receptors and receptor tyrosine kinases

34. Cyclic AMP
Cyclic AMP (cAMP) is one of the most widely used second messengers
Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal
Many signal molecules trigger formation of cAMP
Other components of cAMP pathways are G proteins, G-protein-linked receptors, and protein kinases
cAMP usually activates protein kinase A, which phosphorylates various other proteins
Further regulation of cell metabolism is provided by G-protein systems that inhibit adenylyl cyclase

35. Pyrophosphate ATP Cyclic AMP AMP Phosphodiesterase Adenylyl cyclase

36. First messenger
(signal molecule
such as epinephrine)
Adenylyl
cyclase
G protein
G-protein-linked
receptor
GTP
ATP
Second
messenger
cAMP
Protein
kinase A
Cellular responses

37. Calcium ions and Inositol Triphosphate (IP3)
Calcium ions (Ca2+) act as a second messenger in many pathways
Calcium is an important second messenger because cells can regulate its concentration
A signal relayed by a signal transduction pathway may trigger an increase in calcium in the cytosol
Pathways leading to the release of calcium involve inositol triphosphate (IP3) and diacylglycerol (DAG) as second messengers

38. EXTRACELLULAR Signal molecule FLUID (first messenger) G protein DAG
GTP
G-protein-linked
receptor
PIP2
Phospholipase C
IP3 (second
messenger)
IP3-gated
calcium channel
Endoplasmic
reticulum (ER)
Ca2+
CYTOSOL

39. EXTRACELLULAR Signal molecule FLUID (first messenger) G protein DAG
GTP
G-protein-linked
receptor
PIP2
Phospholipase C
IP3 (second
messenger)
IP3-gated
calcium channel
Endoplasmic
reticulum (ER)
Ca2+
Ca2+
(second
messenger)
CYTOSOL

40. EXTRACELLULAR Signal molecule FLUID (first messenger) G protein DAG
GTP
G-protein-linked
receptor
PIP2
Phospholipase C
IP3 (second
messenger)
IP3-gated
calcium channel
Cellular
re-
sponses
Various
proteins
activated
Endoplasmic
reticulum (ER)
Ca2+
Ca2+
(second
messenger)
CYTOSOL

48. 一開始先有訊息認知(signal perception)作用，包括賀爾蒙訊息和接受子(receptor)間辨識，接著靠second messengers 調控細胞內Ca，一般常見second messenger—inositol phosphates 和 ROS。
利用一protein phosphorylation cascade，將原始訊息放大，
最後標的蛋白(target protein)直接參與細胞層級的保護作用或 transcriptional factors所調控的特定逆境調節反應的基因組群。這些基因產物多與調控分子(e,g, ABA, ethylene, and salicylic acid)生成有關。這些分子可以經由不同組成分參與，而開始第二輪的訊息傳導。
訊息傳導需要所有訊息傳導分子間適當的時空協調。因此有些分子參與modification, delivery and assembly of signaling components，而不是直接和訊息接力傳遞有關，但能確定準確的傳送。這些分子包含protein modifer(enzyme for proteim lipidation, methylation,glycosylation, and ubiquitination), sacafold and adaptors。