This passage is adapted from Julia Karow, “Cell Membrane Exclusivity.” ©2000 by Scientific American.
Like an opera house, which has its public entrance separate
from that for the performers, a cell has different doors for
different molecules. Each gets scrutinized at its door before it
can enter the cell. Now researchers from the University of
California at San Francisco have revealed in the journal
Science the three-dimensional structure of one such door, or
membrane channel, that specializes in granting entry to a
membrane component known as glycerol. Specifically the
channel is called the glycerol facilitator (GlpF), from the
bacterium Escherichia coli.
Bearing three alcohol groups, glycerol is a basic building
block for the cell membrane. (Other components include fatty
acids and small charged molecules.) And not just in E. coli.
Indeed, although the channel the researchers studied is from a
bacterium, it belongs to a large protein family dubbed the
aquaporins, which are found in species ranging from bacteria to
humans.
GlpF is highly specific for glycerol and similar
polyalcohols. Somehow, even though water molecules are
much smaller, they cannot enter. The new study reveals why.
In order for glycerol to clear the four-channel configuration in
the cell membrane, it must pass through a narrow selectivity
filter in the center of a channel. Here it is surrounded by amino
acids that closely match its own structure, which is hydrophilic
(“water-loving”) on one side and hydrophobic (“water-
fearing”) on the other. Water molecules, in contrast, can only
pass through this area in single file, which is not energetically
favorable, because they like to bond to one another. And ions,
which are charged, are unable to pass the “water-fearing” side
of the channel. This cell entrance, it seems, is truly exclusive.
Like an opera house, which has its public entrance separate
from that for the performers, a cell has different doors for
different molecules. Each gets scrutinized at its door before it
can enter the cell. Now researchers from the University of
California at San Francisco have revealed in the journal
Science the three-dimensional structure of one such door, or
membrane channel, that specializes in granting entry to a
membrane component known as glycerol. Specifically the
channel is called the glycerol facilitator (GlpF), from the
bacterium Escherichia coli.
Bearing three alcohol groups, glycerol is a basic building
block for the cell membrane. (Other components include fatty
acids and small charged molecules.) And not just in E. coli.
Indeed, although the channel the researchers studied is from a
bacterium, it belongs to a large protein family dubbed the
aquaporins, which are found in species ranging from bacteria to
humans.
GlpF is highly specific for glycerol and similar
polyalcohols. Somehow, even though water molecules are
much smaller, they cannot enter. The new study reveals why.
In order for glycerol to clear the four-channel configuration in
the cell membrane, it must pass through a narrow selectivity
filter in the center of a channel. Here it is surrounded by amino
acids that closely match its own structure, which is hydrophilic
(“water-loving”) on one side and hydrophobic (“water-
fearing”) on the other. Water molecules, in contrast, can only
pass through this area in single file, which is not energetically
favorable, because they like to bond to one another. And ions,
which are charged, are unable to pass the “water-fearing” side
of the channel. This cell entrance, it seems, is truly exclusive.