Signal Transduction-1 (17!1!2013) BY CHANDI CHARAN SINGH MANDEL .

7/27/2019 Signal Transduction-1 (17!1!2013) BY CHANDI CHARAN SINGH MANDEL .

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Syllabus of Signal Transduction

L T P: 3-1-2 Credit: 5

Unit I: G Protein-Coupled Signaling Pathways (10)Classification, structure and regulation of transmembrane receptors; G-protein-coupled receptors;

Structure, ligand binding and signal transmission; Regulatory GTP-ases; Heterotrimeric G proteins;

Second messengers: cAMP and cGMP.

Unit II: Protein Kinases (10)

Classification and characteristics of protein kinases; Protein phosphorylation and

dephosphorylation; Regulation of Protein Kinase A pathway, Phosphatidyl-3-kinase (PI3K)/Akt

pathway; Janus Kinase and Signal Transducer and Activator of Transcription (JAK-STAT) pathway;Protein cascades of MAPK; Organization of MAPK pathways.

Unit III: Phosphatase and their Types (10)

Role of phosphatase in signal transduction; Types of phosphatase: Tyrosine, serine/threonine, dual

specificity phosphatase and their physiological relevance.

Unit IV: Apoptosis (10)

Classification and functions of caspases; Intrinsic and Extrinsic death pathways; Bcl-2 protein

family; Role of p53 in apoptosis.

Reference Books:

Krauss G., Biochemistry of Signal Transduction andRegulation,Wiley-VCH, 2008.

Hancock J.T., Cell Signalling, Oxford University Press, 2010.

Gomperts B.D., Kramer I.M. and Tatham P.E.R., SignalTransduction, Academic Press, 2009.


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The enormous structural varieties and functional capacities

of multicellular organisms are due to their ability tocoordinate the biochemical reactions of the various cells of

the whole organisms.

The basis for this coordination is the intercellular

communication, which allows a single cell to influence the

behavior and functional property of other cells in a specificmanner.

General Function of Signal Pathways

Several Ways of Intercellular Communication

Chemical Messengers

Gap Junctions

Cell Surface Proteins

Intercellular Communication by Electrical Impulses


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Chemical Messengers:

Cells send out signals in the form of specific chemical messengers and thetarget cells receive these signals and transmits into biochemical reactions.

Signaling cells can simultaneously influence many cells by chemical

messengers so as to enable a coordinated reaction in an organism.

Gap Junctions:

Communication between bordering cells is possible via direct contact inthe form ofgapjunctions. Gap junctions are channels that connect two

neighboring cells to allow a direct exchange of metabolites, signaling

molecules and other molecules between the cells.

Cell-cell interaction via cell surface proteins:

Direct communication between cells can occur with the help of cell surfaceproteins. In this process a cell surface protein of one cell binds to a specific

complementary protein on another cell. These bindings activate specific

intracellular signaling cascade which eventually initiates specific

biochemical reactions in the participating cells.

Several Ways of Intercellular Communication


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Chemical Messengers

Principal Mechanisms of Intercellular Communication

Schematic Diagram of Intercellular Communication

Gap-Junction Cell Surface Proteins
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Chemical Messengers

Local Chemical Messengers such

as growth Factor- TGF- Beta,

(Paracrine and Autocrine).

Neurotransmitters (Dopamine, NO)

Hormones (these are long-range

chemical messengers secreted into

blood by endocrine gland.

Neurohormones (Vasopressin)


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Several steps of intercellular communication


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Receptor and Signal Transduction

Receptors (radar) are proteins.

containing a binding site specific for a single chemical

messenger and another binding site involved in

transmitting the message.

The second binding site may interact with anotherprotein or with DNA (for steroid hormone).

Type:

Plasma membrane (Transmembrane) receptors Nuclear receptors


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Transmembrane membrane receptor:

Transmembrane receptors are a class of protein which

span the plasma membrane by transmembrane domain

and contain an extracellular binding domain for

receiving the messenger and contain a cytosolic

domain for transducing the message.Nuclear receptors:

Nuclear receptors are a class of proteins found within

cells that are responsible for sensing steroid and thyroid

hormones and certain other molecules. Nuclearreceptors have the ability to directly bind to DNA and

regulate the expression of adjacent genes, hence these

receptors are classified as transcription factors.


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Several categories of Transmembrane

membrane receptor:

Chemically gated receptors or Ion channel receptors

Receptor enzyme kinases (Tyrosine kinase receptor

and Serine-threonine kinase receptors).

G-protein coupled receptors


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Chemically gated receptors or Ion channel receptors
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Receptor Enzyme

Receptor Enzyme Kinases


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G-protein coupled receptors


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Nuclear receptors

Receptor


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Signaling via Transmembrane Receptors

Transmembrane receptors are proteins that span the phospholipid bilayer

of the cell membrane.

The signaling molecule (such as chemical messenger) binds on the

extracellular side to the receptor. This binding activates receptor protein.

Activated receptor proteins transduce the signal to the effector protein,

the next component of the signal transmission pathway on the inner side of

the cell membrane.

In this process enzymatic activities can be triggered and/or the activated

receptor engages in specific interactions with downstream signal proteins.


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Structural Principles of Transmembrane Receptors

The ExtracellularDomain of Transmembrane Receptors

The Transmembrane Domain

The IntracellularDomain of Membrane Receptors

Regulation of Receptor Activity


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Structural principles of transmembrane receptors.

Representation of the most important functional

domains of transmembrane receptors


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Examples of subunit structures. Transmembrane receptors

can exist in a monomeric form (1), dimeric form (2) and ashigher oligomers (3,4). Further subunits may associate at

theextracellular and cytosolic domains, via disulfide

bridges (3) or via non-covalent interactions (4).


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Examples of structures of the transmembrane domains of

receptors.

The transmembrane domain may be composed of an a-helix (1) orseveral a-helices linked by loops at the cytosolic and extracellular

side (2). The 7-helix transmembrane receptors are a frequently

occurring receptor type. Several subunits of a transmembrane

protein may associate into an oligomeric structure (3),



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The Extracellular Domain of Transmembrane Receptors

The extracellular domain often contains the ligand-binding site.

Glycosylation sites, i.e. attachment sites for carbohydrate

residues, are also located nearby in the extracellular domain.

The extracellular localized protein portion may be formed from

a continuous protein chain and may include several hundred

amino acids. If the receptor crosses the membrane with several

transmembrane segments, the extracellular domain is formed from

several loops of the protein chain that may be linked by disulfide

bridges. The structure of the extracellular domain can be very diverse

and is determined by the number of transmembrane sections as

well as the subunit structure of the receptor.


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The transmembrane domains have different functions,

according to the type of receptor.

For ligand-controlled receptors, the function of the

transmembrane domain is to pass the signal on to the cytosolic

domain of the receptor.For ligand-controlled ion channels, the transmembrane portion

forms an ion pore that allows selective and regulated passage of

ions.

The transmembrane receptors span the 5nm thickphospholipid bilayer of the cell membrane with structural

portions known as transmembrane elements.

the transmembrane elements include 2030 mostly

hydrophobic amino acids.


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Structure of Transmembrane Elements

High-resolution structural information about thetransmembrane elements of transmembrane receptors

could recently be obtained on the example of rhodopsin,

the light-activated G protein coupled receptor of the vision

process.These data, together with earlier data on the structures

of other transmembrane proteins (e.g., bacteriorhodopsin),

have confirmed that a-helices are the principal structural

building blocks of the transmembrane elements of

membrane receptors.


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Three-dimensional structure of rhodopsin. Two views of rhodopsin. A) The seven a-helices of the G protein-coupled receptor rhodopsin weave back and forth through

the membrane lipid bilayer (yellow lines) from the extracellular environment (bottom)

to the cytoplasm (top). The chromophore 11-cis retinal (yellow) is nested among the

transmembrane helices .B) View into the membrane plane from the cytoplasmic side

of the membrane. Roman numerals indicate numbered helices.


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In addition to a-helices, proteins also use b-structures to cross themembrane. The transmembrane domain of the bacterial OmpF

porin is made up ofb-elements .The b-elements, in this case, are

not mostly made up of hydrophobic amino acids and form abarrel-like structure.

Structure of Transmembrane Elements


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The OmpF porin from Eschericia coli is an integral membrane channel-formingprotein which spans the outer membrane in Gram-negative bacteria. The structure

of a monomer of the OmpF porin is shown. In total, 16 b-bands are configured in

the form of a cylinderand form the walls of a pore through which selective passage

of ions takes place.


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The Intracellular Domain of Membrane Receptors

Two basic mechanisms are used for conduction of the signal tothe inner side of the membrane.

1. * Via specific protein-protein interactions, the next

protein component in the signal transmission pathway, the

effector protein, is activated.* The conformational change that accompanies the

perception of the signal by the receptor creates a new

interaction surface for proteins that are located downstream

of the receptor.

* In the absence of the signal, this interaction surface is

not available. Signal transmission therefore strictly

depends on signal perception by the receptor, and

activation of the effector molecule must be preceded by

activation of the receptor by a signal.v


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2. * Arrival of the signal triggers enzyme activity in thecytosolic domain of the receptor.

The Intracellular Domain of Membrane Receptors

* The enzyme activity of the cytosolic domain is

often tyrosine kinase activity; however, there are other

examples where tyrosine phosphatase or Ser/Thr-specific protein kinase activity is activated.

* In all these examples, the cytoplasmic domain

carries an enzyme activity regulated by ligand binding.

* The enzyme activity may be an integral part of thereceptor, or it may also be a separate enzyme associated

with the receptor on the inner side of the membrane.


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General functions of transmembrane receptors. Extracellular signals convert the

transmembrane receptor from the inactive form R to the active form R*. The activated receptor

transmits the signal to effector proteins next in the reaction sequence. Important effector

reactions are the activation of heterotrimeric G-proteins, of protein tyrosine kinases and of

protein tyrosine phosphatases. The tyrosine kinases and tyrosine phosphatases may be an

intrinsic part of the receptor or they may be associated with the receptor. The activated

receptor may also include adaptor proteins in the signaling pathway or it may induce openingof ion channels.

General functions of transmembrane receptors


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Regulation of Receptor Activity

These are protein sequences that permit

phosphorylation of the receptor by protein kinases.

Phosphorylation at Ser/Thr or Tyr residues of the

cytosolic domain may lead to activation of the receptor

and thus strengthen signal transmission.

In this way, the protein kinases involved are often part ofother signaling pathways and can link the activity of the

transmembrane receptors to other signaling networks.


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G protein coupled receptors

Interact with G proteins.

Has 7 transmembrane receptors(7TM).

Single largest super family of proteins encoded byanimal genome.


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G PROTEIN COOUPLED

RECEPTORS


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Topology

Amino terminus faces outside of thecell

Carboxy terminus faces inside of the

cell Has 7 -helices


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Signal transduction by GPCRs


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Activation cycle of a G-protein by a G-protein-

coupled receptor receiving a ligand.
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G PROTEIN COOUPLEDRECEPTORS

Signal Transduction-1 (17!1!2013) BY CHANDI CHARAN SINGH MANDEL .

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Transcript of Signal Transduction-1 (17!1!2013) BY CHANDI CHARAN SINGH MANDEL .