Let's step back for a moment and reconsider what
a protein kinase is. A protein kinase is an enzyme
that phosphorylates other proteins. Sometimes as
we've previously seen, these protein kinases
are intracellular protein kinases. Other times,
the protein kinase is actually embedded in the
cell membrane. Receptor tyrosine kinases are a
perfect example of these embedded protein kinases.
So here we have a protein that is about to be
phosphorylated by a protein kinase.
Recall the protein kinase is going to take its
phosphates off of either ATP or we've seen GTP
in our examples of G-proteins. And it's going to
form ADP but stick the phosphate right on to this
protein. So protein kinase an enzyme that
phosphorylates any other protein.
We can also have dephosphorylation which will
inactivate. In the last lecture we looked at how
we could turn things on with phosphorylation or
we could actually turn things off with phosphorylation.
The example might be if we phosphorylate a protein
with protein kinase and it activates that cascade,
or we could phosphorylate a protein with a protein
kinase. And that phosphorylated protein could perhaps
go and grab one of the intermediates and stop it
working in the cascade. Much like taking out a
firemen and not letting him do his job. So, the
other thing that can happen is we can have this
protein that's been phosphorylated, recycled. Its
been dephosphorylated because usually the
phosphorylated protein is going to transfer its
phosphate to another protein. And then that one
to another protein. And so on and so forth down the
cascade until we actually have our full blown
cellular response. Recall that I mentioned receptor
tyrosine kinases are involved in general cell
functions. The everyday life of a cell. Things like
cell cycle and cell growth. Cell migration
from place to place. Cell metabolism, cell proliferation
and many growth factor regulation processes.
So growth factors often work through receptor
tyrosine kinases to form their effects
in growth and development. Before we get into the
details of how these receptors work and
what happens in the cascade, let's look at the
anatomy of a receptor tyrosine kinase.
What you see here are two receptor tyrosine kinases.
They each have a single transmembrane domain,
the alphahelical portion. They also have a receptor
ligand binding domain on the outside of the cell.
And on the inside of the cell we have the actual
receptor tyrosine kinase protein. So here,
the protein kinase is attached to the receptor that's
on the outside of the cell. Rather than having an
intracellular protein kinase which is what we've
looked at previously. So once a ligand binds
to the receptor portion of the receptor
tyrosine kinase, we'll see dimerization.
That is these two subunits come together. So as
they come together, they phosphorylate each other.
Recall that the portion dangling inside the cell is
the actual protein kinase portion. And because it's
a protein kinase it can take phosphates off of ATPs
and stick them on the other receptor tyrosine kinase.
So in this fashion they diamerize, they join together.
The phosphotyrosine regions, the tyrosines with the
phosphates on the side actually act as docking sites
for other proteins involved in the signal transduction
cascade. The response that we get from a receptor
tyrosine kinase diamerization is completely dependent
on the type of response proteins that
are involved. Most often these are enzymes.
In order for proteins to dock or these enzymes to
dock on to the phosphotyrosines, we often need
docking proteins. So docking proteins will help
other enzymes dock on to the phosphotyrosine sites.
In addition to docking proteins, sometimes we need
adapters. Sort of like when you're plugging in
electricity in a foreign country. You need an adapter
to make sure it fits directly on to this particular
prongs or phosphates that are sticking out of the
phosphotyrosines. So once these two are diamerized
and we've started to activate that protein cascade,
there are a multiple different options of things
that will happen. Shortly we'll take a look at a
couple of great examples of a cascade that could
result from receptor tyrosine kinase activation.
So in the case of insulin, it binds to the
extracellular receptor domain. And then it activates
the receptor tyrosine kinases. They autophosphorylate
each other. And then we have the insulin response
protein that's going to bind; It's a docking protein,
it's going to bind to those phosphate prongs on the
receptor tyrosine kinase. Following that it's going
to pass the phosphates on to other proteins that
then result in activating glycogen synthase.
So we're going to phosphorylate a bunch of proteins.
Essentially that result in transcription and
translation of things that make glycogen synthase.
And then glycogen synthase is going to take
glucose molecules and stick them togeher to form
glycogen. So in this sense, insulin is reducing
the sugar that's floating around and free and ready
for metabolism and stuffing it into glycogen molecules
and thus reducing blood sugar and increasing the
storage of carbohydrates. So how is amplification
of these signals actually accomplished. There is a
great example by looking at mitogen activated
protein kinase cascades or MAP cascades. In which
kinases are activated by a series of protein kinases.
So this mitogen kinases are stepwise activated. And
in the diagram we saw earlier you can see that
each step along the way there is potential
for amplification of the signal.
First of all we have a module of protein kinases.
And we'll see how these are associated shortly.
And they will phosphorylate each other. So here we have
an example of the phosphorylation being passed down
through each of these activating kinases. It's not
important for us to know precisely what those are
but just get the general scheme of how each
step of the way we're passing the phosphate on
and we could be amplifying the signal cause there's
more protein kinases and thus more phosphorylation.
So finally we get the phosphorylation of MAP kinase.
And as we're going to see shortly MAP kinase
plays a particular role in linking things together
to the cellular response. So again as we move through
each step of that mitogen activated kinase cascade,
we can see that this signal can be amplified
further and further and further until we eventually
get a cellular response. I mentioned that these
were grouped in a module. This is because we have
things called scaffold proteins. They're essentially
a protein that holds each of the protein kinases
involved in this mitogen activating kinase cascade
in one protein subunit. Otherwise, you'd have these
things freely floating around the cell and it would be
really hard to get them together. So scaffold proteins
hold groups of related proteins in a pathway together
so that we can keep the response fairly localized.
The cellular response could result
in many possible targets. We could be increasing the
rate of transcription and translation.
Phosphorylating transcription factors so that they
can dock on to the DNA and help activate that process.
We'll look at what some of those specific proteins
are in a future lecture. But activating gene expression
is one of the keys of these receptor tyrosine kinase
pathways. Molecular switches also link
receptor tyrosine kinases to MAP kinase cascades.
Let's take a look at an example of a molecular switch.
These switches can link external signals to internal
transduction pathways and they are sometimes broken
and that can result in cancer. Again when we
look at cell division and cell cycle controls,
we'll be able to see one of those mechanisms in play.
So receptor tyrosine kinases are often linked to
protein kinase cascades by molecular switches.
Here is an example of a molecular switch.
Often the molecular switches are activated by
external cell signals at another receptor location.
We'll see how they come together shortly. So we have
this external signal in this case activating
guanine exchange factors. Guanine exchange factors
will take GTP, guanine triphosphate and take the
phosphates off transferring them to another protein.
In this case, we're looking at RAS. RAS is activated
by the transfer of GTP onto itself. And it is
inactivated when we remove the GTP.
The RAS class of proteins is a pretty active area
of research at this moment, but here we can see
precisely how it works as a molecular switch. So
again another receptor activates guanine exchange factors
which help phosphorylate the protein RAS or
dephosphorylate the protein RAS. So when we remove
the ATP, RAS is deactivated. And when we add the GTP,
RAS is activated. So here is a summary of how
a receptor tyrosine kinase pathway might work including
the receptor tyrosine kinase itself, the MAP cascade,
and our RAS protein molecular switch. So we have,
RAS as a switch in the middle and that links
the receptor tyrosine kinase to the MAP kinase cascade
that we've previously examined. And that will then
impact what sorts of cellular responses we get by
activating a variety of different transcription factors.
Again the response we get depends entirely upon which
proteins are activated, which gets quite complex
because that depends on all sorts of other receptors
coming in to the cell. So again, how do the cells
really decide what it is they want to do.
Well they have to choose based on what the DNA says
which people in the room they're listening to. If you
recall the analogy of having a room full of people.
It's very loud and bustling. How do you
communicate with the people right next to you.
The cell gets to choose which signals that it listens
to. By depending on which receptors are in its
cell membrane. So as you can see there are multiple
levels for modulation of these signal transduction
systems. The more players there are, the more complex
it is. In fact to me, it's often surprising
that any of it works out at all.