- We will look at the ABC transport proteins, then examples of ion channels that
are a distinct category from transport proteins but also involved in moving small
molecules across membranes and we’re going to look at how these ion channels
contribute to the membrane potential in animal cells and also look at the
different categories of ion channels in terms of gated ion channels.
- Here is an overview of the different types of transport proteins that we’ve
looked at so far. We looked at passive transport carried out by uniporters, active
transport carried out by coupled transporters and particularly, the symporters
and antiporters. Then we looked at ATP driven pumps, these also carry out
active transport and in this case, it is primary active transport.
- We also looked at the P-type ATPases that are phosphorylated, V type ATPases
that mainly pump protons across membranes, F-type ATPases that are involved
in ATP synthesis and then today we’re going to look at the final category of
transport proteins: the ABC transporters.
! Dimerization of ATP binding domains
! Conformational changes in transmembrane domains
- Here is an overview of what ABC transporters look like. They’re called ABC
transporters b/c they have 2 ATP binding cassettes (domains). ATP binding
cassettes would be ABC so it says domains but they’re also called ATP binding
cassettes & in this overview, there are 2 ATP binding cassettes that are found on
the cytosolic side of the membrane. Those can be the ATP binding domains,
then there are two transmembrane domains in these proteins.
- They’re found in both bacterial and eucaryotic cells, the main difference b/w
these 2 figures is that in bacterial cells, they’re mainly carrying out nutrient
uptake from the environment & in eucaryotic cells, they’re typically carrying
out the transport of toxins & waste products outside of the cell. This is not
exclusively true, there are some bacterial ABC transporters that will also export
toxins outside of the cell but one of the main functions is to uptake nutrients.
- How is this done? One of the first steps is ATP binding. On the left is the
original conformation of the ABC transporter, a small molecule, which could be
a nutrient from the environment, will bind to the ABC transporter since this
pocket here where the molecule binds is exposed to the outside of the cell.
- Then the ABC transporter will bind ATP, there are 2 ATP binding domains, so
this ABC transporter will bind 2 ATP molecules and this will induce a
conformational change in this protein. This is important, again this is a
transporter protein and one thing that unifies these transporter proteins is that
they all undergo conformational changes as they’re transporting small molecules
across the membrane.
- You have ABC transporter there, ATP binds & induces a conformational
change, then the ATP is hydrolyzed to ADP & that exposes this small molecule
now to the cytoplasm of the cell & it will be released in the cytoplasm. That is
how basically the overview of how ATP binding & ATP hydrolysis induces this
- Recap: ATP binding induces the dimerization of ATP binding domains, so you
can see that those are the two ATP binding domains, they aren’t associated with
each other, when they bind to ATP, they dimerize together inducing a
conformational change. Then the ATP is hydrolyzed shown in the slide and this
causes the dissociation of the small molecule from the ABC transporter and
ATP binding and ATP hydrolysis induces these conformational changes in the
transmembrane domains as shown in the slide. This exposes the binding site of
small molecules, it goes from being exposed to the extracellular space and then
the conformational change induces it to be exposed to the cytosol of the cell.
The substrate binding site goes from being exposed on one side of the
membrane, to the other and that induces transport.
- Now when the eucaryotic ABC transporter that would be exporting toxins out of
the cell, the original conformation would have substrate binding sites on the
cytosolic side, the small molecule can bind, ATP binding to the ABC transporter
would induce dimerization of these domains, and induce a conformational
change of the transmembrane domains. ATP hydrolysis would induce the
dissociation of the small molecule now in this case to the outside of the cell.
! Sequester toxins – removed by leaf shedding
- In bacteria, these ABC transporters are important for importing nutrients into
the cell but they’re also involved in exporting toxins and they can actually be
very important in developing antibiotic resistance in bacteria since some of
these ABC transporters can actually transport antibiotics outside of bacterial
cells and then these bacteria can become more tolerant to antibiotics. So they
can be involved in import and export in bacteria.
- In plants, they are involved in transporting toxins from the cytosol to the
vacuole & this is done to store these toxins in the vacuole & then they can be
removed by leaf shedding. So the toxins become sequestered in the vacuole by
the ABC transporters & then plant can get rid of these toxins by shedding its
- In animals, they’re also used to export natural toxins & also waste products &
also drugs. One of the very famous examples is called multi-drug resistant
protein 1 or MDR1. This protein can export drugs from cancer cells and there is
over-expression of MDR1 in numerous types of cancers and this can lead to
resistance of these cancers to chemotherapeutic drugs. Since the cancer
overproduces these ABC transporters called MDR1, the chemotherapeutic drugs
are pumped out of the cancerous cell & are much less efficient at treating
- This is a diagram of MDR-1 and normally it's expressed in liver, kidney and
intestinal cells. It is involved in excreting natural toxins or waste products into
the bile, urine or feces but it is also able to export drugs so its ability to excrete
toxins also make it capable of exporting drugs and it is overexpressed in many
cancer lines following treatment with chemotherapeutic drugs. This leads to
resistance of cancers to chemotherapy.
- ABC transporters play very important roles in numerous drug resistance,
antibiotic resistance and we’re not going to cover it in detail but it is also the
mutation of ABC transporters that can lead to cystic fibrosis. So they play
important role in diseases and drug resistance.
- That is as far as we will cover the transporter proteins, so we just finished the
ABC transporters and now we’re going to move onto a second category of
proteins that move small molecules across membranes: channel proteins. All
channel proteins carry out passive transport and unlike transporter proteins,
channel proteins do not undergo conformational changes as they’re transporting
their small molecules across the membrane.
" Weaker interactions with solute
" Faster transport by channels
" Several molecules pass through when open
- Here is a diagram of a channel protein and basically what it does is it creates a
hydrophilic pore across the membrane, so the grey is the lipid bilayer, then there
is the channel protein in green and it creates this hydrophilic pore through which
these small molecules can cross the membrane.
- Most channel proteins are selective meaning they will transport one particular
type of ion, so you’ll have potassium channels, you’ll have sodium channels,
you’ll have calcium channels and they will be specific for that type of ion.
We’re going to go mainly into different ion channels. These are the major
examples we’re going to cover.
- They all carry out passive transport, now they create a weaker interaction with
their solutes than transporter proteins, so you’ll recall that the glucose uniporters
also carried out passive transport but they bind to one molecule of glucose,
undergo a conformational change, release the glucose on the other side of the
membrane and then change conformation back to be able to transport another
molecule of glucose. You can see that that it would be much slower than
transporting a molecule just allowing it traverse the membrane through a
channel. They have much weaker interaction with the solute, and they carry out
faster transport than transporter proteins that carry out passive transport. There
is much faster transport by channels than by transporter proteins.
- Another important feature is that several molecules can cross ion channels
simultaneously. When the channel is open, multiple small molecules can cross
through the channel. This is in contrast to the uniporter which binds to one
molecule and transports one molecule at a time across the membrane.
- Ion channels are found in animals and in plants and in microorganisms so they
are found throughout the different kingdoms and there are two major types.
- There are the non-gated ion channels that are always open. The major example
we’ll cover is the potassium leak channels & what we’re going to cover is that
they play a major role in generating the resting membrane potential in animal
cells. These channels are non-gated channels and they’re always open.
- The 2nd category are gated ion channels and these are not always open, they
need to be opened by an external stimulus such as a chemical or an electrical
signal which is required for this channel to open.
" Always open
" K+ moves out of the cell
- Now the first example we’re going to go through are the non-gated ion channel
and particularly the potassium leak channel.
- The potassium leak channel is found in the plasma membrane of animal cells
and they’re always open. These are the non-gated channels.
- As you’ll recall, potassium is in higher concentration in the cytosol relative to
the outside of the cell so what happens is the potassium moves out of the cell
down its concentration gradient.
- Remember that the Na-K pump is pumping Na outside of the cell and K inside
of the cell and there is a buildup of K inside of the cell relative to the outside.