Alterations in these signalling pathways cause many human
diseases including cancer, diabetes and immune defects. In order to illustrate
the variety of mechanisms used to activate key transcription factors, the eight
classes of cell surface receptors and the intracellular signal pathways that
they activate area important. Ligand binding to many receptors causes
activation by inducing two or more receptor molecules to form a complex on the
cell surface. Most signalling pathways involve one or more protein Kinases.
Many extracellular signaling molecules are synthesized and
released by signaling cells within the organism. In all cases signaling
molecules produce a specific response only in target cells that have receptors
for the signaling molecules. Many types of chemicals are used as signals: small
molecules, peptides, soluble proteins and many proteins presented to the
surface of a cell or found to the extracellular matrix. Most receptors bind a
signal molecule are a group of closely related molecules.
Most signaling molecules, however are too large and too
hydrophilic to penetrate through the plasma membrane. These bind to cell
surface receptors that are integral proteins on the plasma membrane.
Cell surface receptors
generally consists of three discrete segments
Ø a segment on the extracellular
Ø segment that spans the plasma
membrane and a segment facing the cytosol.
Most ligands are responsible for cell-cell signaling
(including neurotransmitters, peptide hormones, and growth factors) bind to
receptors on the surface of their target cells. A major aspect in understanding
the cell-cell signaling is understanding the mechanisms by which cell surface
receptors transmit the signals initiated by ligand binding. Cell surface
receptors, including the receptors for peptide hormones and growth factors, act
instead by regulating the activity of intracellular proteins. These proteins
then transmit signals from the receptor to a series of additional intracellular
targets, frequently including transmission factors. Ligand binding to a
receptor on the surface of the cell thus initiates a chain of intracellular
reactions, ultimately reaching the target cell nucleus and resulting in
programmed changes in Gene Expression.
G Protein Coupled Receptors
The largest family of cell surface receptor transmits signal
to intracellular targets via the intermediatary action of guanine nucleotides
binding proteins called G proteins. Nearly thousand such G protein-Cupled
receptors have been identified including the receptors for Eucosanoids, many
neurotransmitters, neuropeptides and peptides hormones. In addition, the G-Protein
coupled receptor family includes a large number of receptors that are
responsible for smell, site and taste
Coupled receptors are structurally and functionally related proteins
characterized by seven membrane-spanning alpha helices. The binding of Ligands
to the extracellular domains of these receptor induces conformational change
that allows the cytosolic domain of the receptors to activate a G-Protein
associated with the inner phases of the plasma membrane
G-Protein then dissociates from the receptors and carries the signal to an
intracellular target, which may be either an enzyme or an ion channels.
G-protein consists of three subunits. They are a,b and
g. They are frequently called heterotrimeric G
proteins to distinguish them from other guanine nucleotide-binding proteins. The
a subunit binds guanine nucleotides, which
regulates G protein activity. In the resting state, a
is bound to GDP in a complex with b and g.
Hormone binding induces conformational change in the receptor. Hence, the
active GTP bound a
subunit and the bg complex
elucidates the intracellular response.
Receptor Tyrosine Kinases
In contrast to the G-Protein Coupled
Receptors, other cell surface receptors are directly linked to intracellular
enzymes. The largest family of such enzyme-linked receptors is the receptor is
the receptor protein-tyrosine kinases, which phosphorylates their substrate
protein on tyrosine residues. This family includes the receptors for most
polypeptide growth factors, so protein tyrosine phosphorylation has been
studied as a signalling mechanism involved in the control of animal cell growth
human genome encodes 59 receptor protein-tyrosine Kinases, including the
receptors for EGF, NGF, PDGF, insulin and many other growth factors. All these
receptors share a common structural organization: an N-terminal domain, a
single transmembrane a
helix and a cytosolic C-terminal
of the receptor protein-tyrosine Kinases consists of single polypeptides. The
binding of ligands to the extracellular domains of these receptors activates
cytosolic Kinases Domains resulting in phosphorylation which leads to
conformational change that promote protein-protein interactions between
different receptors polypeptides.
than possessing intrinsic enzymatic activity, many receptors act by stimulating
intracellular protein-tyrosine Kinase with which they are non-covalently
associated. This family of receptors are called as cytokine receptor
superfamily. It includes receptors for cytokines e.g., interlukin-2 and
erythropoietin and for some polypeptide hormones e.g., growth hormone. Like
receptor protein-tyrosine kinases, the cytokine receptors contain N-terminal
extracellular ligand-binding domains, single transmembrane a helices
and C-cytosolic domains. However, the cytosolic domains of the cytosolic
domains o the cytokine receptors are devoid of any known catalytic activity.
Instead, the cytokine receptors function in association with non-receptor
protein-tyrosine kinases, which are activated as a result of ligand binding.
Kinases associated with cytokine receptors belong to the Janus Kinase or JAK
family, which consists of four related non-receptor protein tyrosine kinases.
Members of the JAK family appear to be universally required for signalling from
cytokine receptors, indicating that JAK family Kinases play a critical role in
coupling these receptors to the tyrosine phosphorylation of intracellular
first step in signalling from cytokine receptors is ligand-induced receptor
dimerization and cross-phosphorylation of the associated non-receptor
protein-tyrosine kinases. These activated Kinases then phosphorylate te
receptor, providing phosphotyrosine binding sites for the recruitment of
downstream signalling molecules that
contain SH2 domains. Combinations of cytokine receptors plus associated
non-receptor protein-tyrosine kinases thus function analogously to the receptor
Growth Factors b ( TGF-??
The receptors for TGF-b and related
polypeptides are protein kinases that phosphorylate serine or threonine, rather
than tyrosine residues on their substrate proteins. TGF-b is the prototype of a
family of polypeptide growth factors that control proliferation and
differentiation of a variety of cell types. The cloning of the first receptor
for a member of the TGF-b family in 1991 revealed that it is the prototype of a
unique receptor family wit acytosolic protein-serine/threonine kinase
domain. Since then, receptors results in
the association of two distinct types of polypeptide chain which are encoded by
different members of the TGf-b receptor family to form hetrodimers in which one
of the receptor kinases phosphorylates the other. The activated TGF-b receptors
then phosphorylate members of a family of transcription factors called Smads
which translocate to the nucleus and stimulate expression of target genes.
The signaling molecules acts as a ligand which binds to a
structurally complementary site on the extracellular or membrane spanning
domains of the receptors. Binding of the ligand induces a conformational change
in the receptor that is transmitted through the membrane spanning domain to the
cytosolic domain resulting in binding to and subsequent activation of other
proteins in the cytosol or attached to the plasma membrane. The overall process
of converting extracellular signals into intracellular responses as well as the
individual steps in these processes is termed as Signal transduction. The molecules
involved in the process are called as Signal transducers.
Intracellular signalling was first elucidated by studies of
the action of hormones such as epinephrine. In 1958 Earl Sutherland discovered
that the action of epinephrine was mediated by an increase in the intracellular
concentration of cyclic AMP ( cAMP) leading to the concept that cAMP is a
Second Messenger in hormone signalling . Cyclic AMP is formed from ATP by the
action of adenylyl cyclase and degraded to AMP by cAMP phosphodiesterase .
Effect of cAMP in animal cells are mediated by the action of cAMP dependant protein kinase or protein kinase. The
inactive form of protein kinase A is a tetramer consisting of tetramer
consisting of two regulatory and two catalytic subunits. Cyclic AMP binds to
the regulatory subunits, leading to their dissociation from the catalylic
subunits. The free catalytic subunits are then enzymatically active and able to
phosphorylate serine residues on their target proteins. The chain of reactions
leading from the epinephrine receptor to glycogen phosphorylase provides a good
illustration of signal amplification during intracellular signal transduction.
Inositol tri phosphate
The inositol 1,4,5-trisphosphate (InsP3)
receptors (InsP3Rs) are a family of Ca2+ release channels present predominately
in the endoplasmic reticulum of all types of cell. Their main role is to
release Ca2+ into the cytoplasm in response to InsP3 produced by several types
of stimuli which leads to generation of complex local and global Ca2+ signals
that regulate numerous cell physiological processes starting from gene transcription to secretion to
learning and memory. The InsP3R is a calcium-selective cation channel whose
gating is regulated by InsP3, and also by cytoplasmic Ca2+. Over the last
decade, detailed quantitative studies of InsP3R channel function and its
regulation by ligands and interacting proteins have provided new insights into
a remarkable richness of channel regulation and of the structural aspects that
underlie signal transduction and permeation.
The modulation of cytoplasmic Ca2+ concentration
by release from internal stores through the inositol trisphosphate receptor
(InsP3R) Ca2+ release channel is a common signalling system involved in the
regulation of several processes. Because of its expression and roles in
regulating several cell physiological processes, InsP3R has been implicated in
a number of disease states. However, relatively few mutations in InsP3R genes
have been identified to date.