- 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