Name: Qurat ul Ain
Class: BS Botany 6th
Topic: Aquaporin In Plants
Submitted to: Sir Ameer Khan
Aquaporin in Plants
Aquaporins are integral membrane proteins from a
larger family of major intrinsic proteins (MIP) that form pores in
themembrane of biological cells.
Genetic defects involving aquaporin genes have been associated
with several human diseases The 2003 Nobel Prize in
Chemistry was awarded jointly to Peter Agre for the discovery of
aquaporins and Roderick MacKinnon for his work on the structure
and mechanism of potassium channels.
membranes of a variety of different animal and plant cellscontain
aquaporins through which water can flow more rapidly inside
the cell than by diffusing through the phospholipid bilayer.
Aquaporins are "the plumbing system for cells," said Agre. Every
cell is primarily water. "But the water doesn’t just sit in the cell, it
moves through it in a very organized way. The process occurs
rapidly in tissues that have these aquaporins or water channels."
For many years, scientists assumed that water leaked through the
cell membrane, and some water does. "But the very rapid
movement of water through some cells was not explained by this
theory," said Agre.
Aquaporins selectively conduct water molecules in and out of the
cell, while preventing the passage of ions and other solutes. Also
known as water channels, aquaporins are integral membrane
pore proteins. Some of them, known as aquaglyceroporins, also
transport other small uncharged solutes, such as glycerol, CO2,
ammonia and urea across the membrane, depending on the size
of the pore. For example, the aquaporin 3 channel has a pore
width of 8-10 Ångströms and allows the passage of hydrophilic
molecules ranging between 150-200 Da. However, the water
pores are completely impermeable to charged species, such
as protons, a property critical for the conservation of the
Water molecules traverse through the pore of the channel in
single file. The presence of water channels increases membrane
permeability to water.
Many human cell types express them, as do certain bacteria and
many other organisms, such as plants for which it is essential for
the water transport system and tolerance to drought and salt
Agre said he discovered aquaporins "by serendipity." His lab had
an N.I.H. grant to study the Rh blood group antigens. They
isolated the Rh molecule but a second molecule, 28 kilodaltons in
size (and therefore called 28K) kept appearing. At first they
thought it was a piece of the Rh molecule, or a contaminant, but it
turned out to be an undiscovered molecule with unknown
function. It was abundant in red blood cells and kidney tubes, and
related to proteins of diverse origins, like the brains of fruit flies,
bacteria, the lenses of eyes, and plant tissues.
In most cells, water moves in and out by osmosis through the lipid
component of cell membranes. Due to the relatively high water
permeability of some epithelial cells it was long suspected that
some additional mechanism for water transport across
membranes must exist. But it was not until 1992 that the first
aquaporin, ‘aquaporin-1’ (originally known as CHIP 28), was
reported by Peter Agre, of Johns Hopkins University
The pioneering discoveries and research on water channels by
Agre and his colleagues resulted in the presentation of a Nobel
Prize in Chemistry to Agre in 2003. In 1999, together with other
research teams, Agre reported the first high-resolution images of
the three-dimensional structure of an aquaporin, namely,
aquaporin-1.Further studies using supercomputer simulations
have identified the pathway of water as it moves through the
channel and demonstrated how a pore can allow water to pass
without the passage of small solutes.
However the first report of
protein mediated water transport through membranes was
by Gheorghe Benga in 1986. This publication that preceded
Agre's first publication on water membrane transport proteins has
led to a controversy that Benga's work was adequately
recognized by neither Agre nor the Nobel Prize Committee. There
is a long history of water pores, starting in 1957.
There have been
many reviews of the history.
Aquaporin proteins are made up of six transmembrane α-
helices arranged in a right-handed bundle, with the amino and the
carboxyl termini located on the cytoplasmic surface of the
membrane. The amino and carboxyl halves of the sequence show
similarity to each other, in what appears to be a tandem repeat.
Some researchers believe that this results from an early evolution
event that saw the duplication of the half-size gene. There are
also five interhelical loop regions (A – E) that form the
extracellular and cytoplasmic vestibules. Loops B and E are
hydrophobic loops that contain the highly, although not completely
conserved, asparagine–proline–alanine (NPA) motif, which
overlap the middle of the lipid bilayer of the membrane forming a
3-D 'hourglass' structure where the water flows through. This
overlap forms one of the two well-known channel constriction
sites in the peptide, the NPA motif and a second and usually
narrower constriction known as 'selectivity filter' or ar/R selectivity
Aquaporins form tetramers in the cell membrane, with
each monomer acting as a water channel. The different
aquaporins contain differences in their peptide sequence, which
allows for the size of the pore in the protein to differ between
aquaporins. The resultant size of the pore directly affects what
molecules are able to pass through the pore, with small pore
sizes only allowing small molecules like water to pass through the
X-ray profiles show that aquaporins have two conical entrances.
This hourglass shape could be the result of a natural selection
process toward optimal permeability. It has been shown that
conical entrances with suitable opening angle can indeed provide
a large increase of the hydrodynamic channel permeability.
Using computer simulations, it has been suggested that the
orientation of the water molecules moving through the channel
assures that only water passes between cells, due to the
formation of a single line of water molecules. The water molecules
move through the narrow channel by orienting themselves in the
local electrical field formed by the atoms of the channel wall.
Upon entering, the water molecules face with their oxygen atom
down the channel. Midstream, they reverse orientation, facing
with the oxygen atom up.
Why this rotation occurs is not entirely clear yet. Some
researchers identified an electrostatic field generated by the two
aquaporin half-helices HB and HE as the reason for the rotation of
water molecules. Others suggested that it is caused by the
interaction of hydrogen bonds between the oxygen of the water
molecule and the asparagines in the two NPA motifs. Moreover,
whether the rotation of water molecules has any biological
significance is still being discussed. Early studies speculated that
the "bipolar" orientation of water molecules keep them from
conducting protons via the Grotthuss mechanism, while still
permitting a fast flux of water molecules.
More recent studies
question this interpretation and emphasize an electrostatic barrier
as the reason for proton blockage. In the latter view, the rotation
of water molecules is only a side-effect of the electrostatic barrier.
At present (2008), the origin of the electrostatic field is a matter of
debate. While some studies mainly considered the electric field
generated by the protein's half-helices HB and HE, others
emphasized desolvation effects as the proton enters the narrow
ar/R selectivity filter
The ar/R (aromatic/arginine) selectivity filter is a cluster of amino
acids that help bind to water molecules and exclude other
molecules that may try to enter the pore. It is the mechanism by
which the aquaporin is able to selectively bind water molecules
(hence allowing them through) and prevent other molecules from
entering. The ar/R filter is a tetrad that is formed by two amino
acid residues from helices B (HB) and E (HE) and two residues
from loop E (LE1 and LE2), found on either side of the NPA motif.
The ar/R region is usually found towards the extracellular
vestibule, approximately 8 Å above the NPA motif and is often the
narrowest part of the pore. The narrow pore acts to weaken the
hydrogen bonds between the water molecules allowing the water
to interact with the positively charged arginine, which also acts as
a proton filter for the pore.
There are thirteen known types of aquaporins in mammals, and
six of these are located in the kidney,but the existence of many
more is suspected. The most studied aquaporins are compared in
the following table:
Type Location Function
Water reabsorption in
response to ADH
Water reabsorption and
In plants water is taken up from the soil through the roots, where it
passes from the cortex into the vascular tissues. There are two
routes for water to flow in these tissues, known as
the apoplastic and symplastic pathways. The presence of
aquaporins in the cell membranes seems to serve to facilitate the
transcellular symplastic pathway for water transport. When plant
roots are exposed to mercuric chloride, which is known to inhibit
aquaporins, the flow of water is greatly reduced while the flow of
ions is not, supporting the view that there exists a mechanism for
water transport independent of the transport of ions: aquaporins.
Aquaporins in plants are separated into five main homologous
subfamilies, or groups.
Plasma membrane Intrinsic Protein (PIP)
Tonoplast Intrinsic Protein (TIP)
Nodulin-26 like Intrinsic Protein (NIP)
Small basic Intrinsic Protein (SIP)
X Intrinsic Protein (XIP)
These five subfamilies have later been divided into smaller
evolutionary subgroups based on their DNA sequence. PIPs
cluster into two subgroups, PIP1 and PIP2, whilst TIPs cluster into
5 subgroups, TIP1, TIP2, TIP3, TIP4 and TIP5. Each subgroup is
again split up into isoforms e.g. PIP1;1, PIP1;2.
The silencing of plant aquaporins has been linked to poor plant
growth and even death of the plant.
The gating of aquaporins is carried out to stop the flow of water
through the pore of the protein. This may be carried out for a
number of reasons, for example when the plant contains low
amounts of cellular water due to drought.The gating of an
aquaporin is carried out by an interaction between a gating
mechanism and the aquaporin, which causes a 3D change in the
protein so that it blocks the pore and, thus, disallows the flow of
water through the pore. In plants, it has been seen that there are
at least two forms of aquaporin gating. These are gating by the
dephosphorylation of certain serine residues, which has been
seen as a response to drought, and the protonation of specific
histidine residues in response to flooding. The phosphorylation of
an aquaporin has also been linked to the opening and closing of
petals in response to temperature.
If aquaporin could be manipulated, that could potentially solve
medical problems such as fluid retention in heart disease and
brain edema after stroke.
There have been two clear examples of diseases identified as
resulting from mutations in aquaporins:
Mutations in the aquaporin-2 gene cause hereditary
nephrogenic diabetes insipidus in humans.
Mice homozygous for inactivating mutations in the aquaporin-0
gene develop congenital cataracts.
A small number of people have been identified with severe or total
deficiency in aquaporin-1. It is interesting to note that they are, in
general, healthy, but exhibit a defect in the ability to concentrate
solutes in the urine and to conserve water when deprived of
drinking water. Mice with targeted deletions in aquaporin-1 also
exhibit a deficiency in water conservation due to an inability to
concentrate solutes in the kidney medulla by countercurrent