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Figure 2.
Fig. 2. (a) Electron-density map for ECF[1]. As discussed
in the text, the backbone atoms of the  [3]  [3] hexamer,
as seen in the structure of the beef-heart mitochondrial enzyme
(10), were placed in the E. coli unit cell by molecular
replacement and then used to calculate phases, which were used
to calculate the map shown. The coefficients are (F[o]-F[c]),
where the F[o] are the amplitudes observed for the ECF[1]
crystals, and the F[c] are the structure factors calculated from
the backbone atoms of the [3] [3] hexamer.
The C^  backbones
of the and subunits
are shown in blue and green, respectively. [In the nomenclature
of Abrahams et al. (10), these are, respectively, the [E] and
[TP]
subunits.] The resolution of the map is 4.4 Å, and it is
contoured at 2.0  , where
is the
root-mean-square density throughout the unit cell. The strongest
features in the map consist of a series of rods, suggesting -helices.
(b) Electron-density map, as in a, with the backbone of part of
the  subunit,
as seen in the structure of beef-heart MF[1] superimposed in
red. was placed
by superimposing the [3] [3] backbone
atoms of MF[1] and ECF[1] and applying the same transformation
to the MF[1] subunit.
As is apparent in the figure, the electron density for the E.
coli enzyme corresponds remarkably well with the -helical
coiled-coil of the subunit,
which lies at the center of the [3] [3] hexamer
and extends below. The present map shows that the two -helices
within the coiled-coil extend several turns beyond the point at
which they can no longer be seen in the structure of beef-heart
MF[1]. (The rod of density labeled G extends beyond that seen in
this figure but lies outside the field of view.) Another -helix,
including residues 77-90 of the subunit,
was seen in the bovine MF[1] structure close to the so-called
DELSEED region of the subunit.
This helix is also shown in red and coincides with rod-like
electron density labeled C. Additional rod-like density features
seen at the bottom of the figure, including that labeled B, are
presumed to correspond to other -helices of
the subunit.
The features labeled A', G', and B' are related by symmetry to
A, G, and B and are associated with an adjacent molecule in the
crystal. Similarly, the feature T' is also associated with
another molecule in the crystal. The density is in the vicinity
of the 25 or so amino-terminal residues that are at the "top" of
the subunit
and are not seen in the structure of beef-heart MF[1] (10). The
presence of this density feature may indicate that some of these
residues are better ordered in the present crystal form. The
electron-density features that are within the region occupied by
the and subunits
are presumed to reflect the fact that the observed structure
factors, F[o], come from the intact ECF[1] particle, whereas the
calculated structure factors, F[c], include only the polyglycine
backbone atoms of the and ![]()
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