Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

channels and the electrical properties of membranes
619
(A) ion in
vestibule
(B) ion in
selectivity
filter
H
H
H
O
H
O
H
H
O
O
H
H
K + O
O Na +
K + O
O Na +
H
H
O
O
H
H
O
O
O
O
H
O
H
O
H
H
Figure 11–25 K + specificity of the
selectivity filter in a K + channel. The
drawings show K + and Na + ions (A) in the
vestibule and (B) in the selectivity filter of
the pore, viewed in cross section. In the
vestibule, the ions are hydrated. In the
selectivity filter, they have lost their water,
and the carbonyl oxygens are placed to
accommodate a dehydrated K + ion. The
dehydration of the K + ion requires energy,
which is precisely balanced by the energy
regained by the interaction of the ion with
all of the carbonyl oxygens that serve as
surrogate water molecules. Because the
Na + ion is too small to interact with the
oxygens, it can enter the selectivity filter
only at a great energetic expense. The
filter therefore selects K + ions with high
specificity. (A, adapted from Y. Zhou et al.,
Nature 414:43–48, 2001. With permission
from Macmillan Publishers Ltd.)
Mechanosensitive Channels Protect Bacterial Cells Against
Extreme Osmotic Pressures
All organisms, from single-cell bacteria to multicellular animals and plants, must
sense and respond to mechanical forces in their external environment (such as
sound, touch, pressure, shear forces, and gravity) and in their internal environment
(such as osmotic pressure and membrane bending). Numerous proteins are
known to be capable of responding MBoC6 to such m11.24/11.23
mechanical forces, and a large subset
of those proteins has been identified as possible mechanosensitive channels, but
very few of the candidate proteins have been shown directly to be mechanically
activated ion channels. One reason for this dearth in our knowledge is that most
such channels are extremely rare. Auditory hair cells in the human cochlea, for
example, contain extraordinarily sensitive mechanically gated ion channels, but
each of the approximately 15,000 individual hair cells is thought to have a total of
only 50–100 of them (Movie 11.9). Additional difficulties arise because the gating
mechanisms of many mechanosensitive channel types require the channels to be
embedded in complex architectures that require attachment to the extracellular
matrix or to the cytoskeleton and are difficult to reconstitute in the test tube. The
study of mechanosensitive receptors is a field of active investigation.
A well-studied class of mechanosensitive channels is found in the bacterial
plasma membrane. These channels open in response to mechanical stretching
of the lipid bilayer in which they are embedded. When a bacterium experiences
a low-ionic-strength external environment (hypotonic conditions), such as
inner helix
ion pore
CLOSED
OPEN
Figure 11–26 A model for the gating
of a bacterial K + channel. The channel
is viewed in cross section. To adopt
the closed conformation, the four inner
transmembrane helices that line the pore
on the cytosolic side of the selectivity filter
(see Figure 11–24) rearrange to close the
cytosolic entrance to the channel.
(Adapted from E. Perozo et al., Science
285:73–78, 1999.)