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

620 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes
(A)
CLOSED (B) OPEN Figure 11–27 The structure of
mechanosensitive channels. The crystal
structures of MscS in its (A) closed and
(B) open conformation are shown. The
side views (lower panels) show the entire
protein, including the large intracellular
domain. The face views (upper panels)
show the transmembrane domains only.
The open structure occupies more area in
the lipid bilayer and is energetically favored
when a membrane is stretched. This may
explain why the MscS channel opens as
pressure builds up inside the cell. (PDB
codes: 2OAU, 2VV5.)
CYTOSOL
CYTOSOL
rainwater, the cell swells as water seeps in due to an increase in the osmotic pressure.
If the pressure rises to dangerous levels, the cell opens mechanosensitive
channels that allow small molecules to leak out. Bacteria that are experimentally
placed in fresh water can rapidly lose more than 95% of their small molecules in
MBoC6 m11.666/11.28
this manner, including amino acids, sugars, and potassium ions. However, they
keep their macromolecules safely inside and thus can recover quickly after environmental
conditions return to normal.
Mechanical gating has been demonstrated using biophysical techniques in
which force is exerted on pure lipid bilayers containing the bacterial mechanosensitive
channels; for example, by applying suction with a micropipette. Such
measurements demonstrate that the cell has several different channels that open
at different levels of pressure. The mechanosensitive channel of small conductance,
called the MscS channel, opens at low and moderate pressures (Figure
11–27). It is composed of seven identical subunits, which in the open state form a
pore about 1.3 nm in diameter—just big enough to pass ions and small molecules.
Large cytoplasmic domains limit the size of molecules that can reach the pore.
The mechanosensitive channel of large conductance, called the MscL channel,
opens to over 3 nm in diameter when the pressure gets so high that the cell might
burst.
The Function of a Neuron Depends on Its Elongated Structure
The cells that make most sophisticated use of channels are neurons. Before discussing
how they do so, we digress briefly to describe how a typical neuron is
organized.
The fundamental task of a neuron, or nerve cell, is to receive, conduct, and
transmit signals. To perform these functions, neurons are often extremely elongated.
In humans, for example, a single neuron extending from the spinal cord to
a muscle in the foot may be as long as 1 meter. Every neuron consists of a cell body
(containing the nucleus) with a number of thin processes radiating outward from
it. Usually one long axon conducts signals away from the cell body toward distant