Cell biology
Communication between cells
- Communication in cells results in three types of effector
proteins
▪ Transcription regulatory protein acts by
altering gene expression: it is a slow
reaction that converts a stem cell into a
differentiated cell
▪ Altered metabolism
▪ Altered shape or movement by acting on
skeletal proteins
- Ways of communication (when a cell does not
communicate anymore, either cell apoptosis occurs or metastasis: where cells grow
independently of their environment, like cancer cells)
▪ contact-dependent signalling occurs mainly in tissues. When you have a
wound, cells lose contact and therefore a growth-stimulating hormone is
released. When the tissue is healed, the release of the hormone is inhibited
and contact between cells is reconstructed
▪ Paracrine signaling is a form of cell-to-cell communication in which a cell
produces a signal to induce changes in nearby cells, altering the behaviour of
those cells. Signaling molecules known as paracrine factors diffuse over a
relatively short distance (local action).
▪ synaptic signalling is a mechanism that nerve cells use to transfer signals.
When the sending neuron fires, an electrical impulse moves rapidly through
the cell, traveling down an axon. When the impulse reaches the synapse, it
triggers the release of ligands called neurotransmitters, which quickly cross
the small gap between the nerve cells. When the neurotransmitters arrive at
the receiving cell, they bind to receptors and cause a chemical change inside
of the cell.
▪ Endocrine signalling is a form of cell communication where hormones are
released into the bloodstream and only the cells that contain a specific
receptor will respond to the signal.
Not all cells respond similarly to the same neurotransmitter! If acetylcholine acts on muscle cells, it
will cause contraction whereas pacemaker cells in the heart will decrease heart rate if acetylcholine
is involved. → where drugs act on! All receptors respond to the same molecule, but all in a different
way so you can interfere between those receptors.
Different types of signal molecules
I. Hydrophobic molecules can pass the cytoplasmic membrane. The signal molecules are
mainly steroid hormones which have a slow rate of reaction.
- Thyroid hormones are steroid hormones that are synthesized in the thyroid gland by iodine.
It is responsible for activation of metabolism. An overproduction of thyroid hormones results
in a high metabolic rate → decreasing weight.
People suffering from a stroma have a lack of iodine which results in no production of thyroid
hormones. The natural response of the body is enlarging the thyroid gland, hoping to
increase the concentration of thyroid hormones in this way.
- Hydrophobic molecules bind to nuclear receptors that
contain a ligand-binding domain to which the steroid
binds. The amino acid sequence of the DNA binding
, domain must conform with the amino acid sequence in the major groove of the ligand, then
the steroid will be accepted by the receptor.
EXAMPLE of nuclear receptors applied in the chicken and rooster:
Chicken:
▪ Oestradiol binds to oestradiol receptor (OR)
▪ OR binds to specific DNA in liver
▪ Induces expression of ovalbumin (main protein in white of egg) •
▪ Transport ovalbumine to oviduct and egg
Rooster:
▪ Has no oestradiol but testosteron
▪ Testosteron binds to testosteron receptor (TR)
▪ TR binds to different part of DNA that encodes for expression proteins of
rooster-behavior
Modified rooster with gene for ovalbumine
▪ Replace DNA-binding domain of TR by DNA binding domain of OR.
▪ Testosteron activates this hybrid receptor that binds to oestradiol receptor
▪ Thus, testosteron induces ovalbumin in liver rooster. But there is still no oviduct, so no eggs!
II. Hydrophilic molecules
a. Ion channels
b. Protein-Protein interactions
- The receptor phosphorylates other receptors and afterwards it is an
intracellular signal protein that can bind. The phosphorylation of other kinases is
called trans-phosphorylation and that activates kinases domains. It creates binding
sites for signaling proteins in the intracellular space.
- The hormone functions as a dimer that links the two receptor kinases by non-
covalent bonds
- SH2 domains allow
proteins containing those
domains to dock to phosphorylated
tyrosine residues on other proteins.
Protein-protein interactions play a
major role in cellular growth and
development. Modular domains,
which are the subunits of a protein,
moderate these protein
interactions by identifying short peptide sequences. These peptide sequences determine the
binding partners of each protein. One of the more prominent domains is the SH2 domain. It
has two cavities that determine if amino acids can bind.
Signal pathway via RAS mechanism
- When Ras is 'switched on' by incoming signals, it subsequently switches on other
proteins, which ultimately turn on genes involved in cell growth, differentiation and
survival. Mutations in ras genes can lead to the production of permanently activated
Ras proteins. As a result, this can cause unintended and overactive signaling inside the
cell, even in the absence of incoming signals.
- Because these signals result in cell growth and division, overactive Ras signaling can
ultimately lead to cancer. The Ras genes in humans are the most common oncogenes
in human cancer.
- The mutation of a molecular switch with G12V mutation will lead to cancer. The mutation
blocks the hydrolysis of GTP making the protein staying active.
, c. G-protein coupled receptors consist of multiple subunits:
▪ Receptor: located in the membrane
and will not leave when the signal
arrives
▪ The G-protein gets activated when
the signal binds to the receptor and
activates the enzyme.
Structure of G-protein coupled receptors (GPCRs)
The structure of the protein results in a barrel: the
barrel goes seven times through the membrane,
where the N-terminal is located outside the membrane and the C-terminal
inside.
- The lipid is attached to the membrane
- The alpha subunit dissociates from the beta-gamma
subunit as soon as the receptor is occupied. GDP will
convert into GTP to activate the domains. The active
subunits will bind to intracellular enzymes that produce
second messengers as…
▪ Cyclic AMP: synthesized from ATP by adenyl cyclase located on the inner
side of the membrane. Adenylate cyclase is activated by a range of
signaling molecules through the activation of adenylate cyclase stimulatory G (Gs)-protein-
coupled receptors. cAMP binds to protein kinases to process all kind of cellular reactions.
▪ Cyclic GMP: synthesized from ATP by guanylyl cyclase.
▪ IP3: It is made by hydrolysis of phospholipase C (PLC). Together with diacylglycerol (DAG), IP3
is a second messenger molecule used in signal transduction in biological cells. While DAG
stays inside the membrane, IP3 is soluble and diffuses through the cell, where it binds to its
receptor, which is a calcium channel located in the endoplasmic reticulum. When IP3 binds
its receptor, calcium is released into the cytosol, thereby activating various calcium regulated
intracellular signals.
▪ DAG
▪ Ca2+
Can be turned on/off by molecular switches!
by phosphorylation protein kinases use ATP to put a phosphate on an
amino acid side chain. Kinases can act on the OH-group of the amino acids
threonine, serine and tyrosine. The additional negative charge causes
proteins to change their conformation into an active form. Protein
phosphatase makes a protein inactive again.
Signalling can also be induced by GTP binding. The binding of GTP to a molecule causes
it to be in active form. Eventually, the molecule itself will hydrolyse the GTP to GDP +
Pi to turn back in inactive state.
, III. Gas molecules are fast responders: they are often radicals which have a loose elektron
that really wants to bind.
- Nitric oxide (NO) is a radical that is
produced by an oxidation reaction
wherein arginine receives elektrons
from the donor NADPH.
- NO functions in very small blood
vessels. When acetylcholine acts on
the endothelial cells, arginine produces nitric oxide. Nitric oxide diffuses among the
membrane and activates guanylyl cyclase. Guanylyl cyclase activates the second messenger
cAMP which causes the smooth vessel to relax → increase of blood flow
The cytoskeleton consists of three types of protein filaments
I. Microtubuli takes care of the intracellular organisation.
- contains the polymerization unit tubulin. Tubulin consists of an α
and β subunit.
- The β unit is at the bottom of the protein and has the relevant GTP
binding site.
- It consists of 13 filaments
- The dimer and filaments are asymmetric!
- The polymerization occurs at the + end as long as GTP is present. If
GTP is hydrolysed to GDP, the filament is unstable and gets
degraded.
Working mechanism:
1. build from asymmetric -tubulin dimer, microtubule filament is also asymmetric
2. growth and degradation at +end; -end is stable
3. no degradation in middle of filament
4. -tubulin binds GTP and is GTPase (it hydrolyses GTP to GDP)
5. filament is stable with GTP at + end, resulting in more polymerisation
6. filament is instable with GDP at +end, resulting in filament crumbling
➔ Catastrophins increase the frequency of catastrophes
➔ Kinesins hold filaments together, even if GTP is not present!
