Biology Reference
In-Depth Information
the F
1
ATPase. It was the unique shape of this particle when imaged by EM that facilitated its
use as a membrane directional marker. To quote Fernandez-Moran '
the elementary particle
(EP), consists of three parts: (1) a spherical or polyhedral head piece (80 to 100
˚
in diameter);
(2) a cylindrical stalk (about 50
˚
long and 30 to 40
˚
wide); and (3) a base piece (40 x 110
˚
).'
All of the stalked EPs faced the same direction (towards the mitochondrial matrix, see
Figure 9.7
). Later it was shown that all three bioenergetic membranes, the mitochondrial inner
membrane, the bacterial plasma membrane, and the thylakoid, all have similar-shaped
ATPases. For each membrane all of the ATPases face in the same direction (inward for the
mitochondrial inner membrane and the bacterial plasma membrane and outward for the
thylakoid). No ATPase has been found to simultaneously face in both directions.
In addition to the classic Fernandez-Moran experiment, there have been countless other
indications that membrane proteins are always 100% asymmetrically distributed across
membranes. Using patch clamp techniques all ion channels have been shown to orient in
only one direction. The activity of many membrane enzymes has been shown to be only asso-
ciated with one side of the membrane. For example, the Na
þ
/K
þ
ATPase hydrolyzes ATP only
from the inside and pumps Na
þ
out of the cell while pumping K
þ
into the cell (Chapter 14).
Hormones and membrane-protein specific antibodies will bind to only one membrane surface.
.
(a)
(b)
(c)
FIGURE 9.7
The Fernandez-Moran experiment. The mitochondrial inner membrane (the Cristae) was imaged
by EM using negative staining with phosphotungstate. Three different perspectives of the same mitochondrial inner
membrane are shown. The mitochondrial elementary particles (EPs), now known to be the F
1
ATPase, are the white
lollipop-shaped objects. Note they are all attached to the same membrane leaflet (the inner leaflet of the mito-
chondrial inner membrane) and face into the matrix. The thick white tube-like space is the space exterior to the
matrix. The matrix is the dark space in which the elementary particles protrude. Reprinted with Permission.
1964,
Rockefeller University Press. Originally published in The Journal of Cell Biology 22:63
100.
[5]
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