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Figure 3.
Figure 3. Electron density from four structures. Electron
density maps were calculated using s[a]-weighted simulated
annealing composite omit map protocols in the program CNS. Atoms
are drawn from residue 125 to 134, near binding site 1 in domain
D2, and the maps are drawn around the atoms. (a) Form M1. The
2.4 Å map is contoured at 1.2s. The strand is in a
location intermediate between C' and D strands. (b) Form M2,
copy A. The 3.2 Å map is contoured at 1.2s. The atoms form
a single turn of a-helix surrounded by random coil and are in
found in an intermediate location between C' and D strands. (c)
Form H1. The 3.2 Å map is contoured at 0.9s. The atoms
have crossed over to the opposite sheet of the Ig domain; they
form b strand D hydrogen-bonded to the E strand. (d) Form M2,
copy B. The 3.2 Å map is contoured at 1.2s. This is the
most common conformation for the Fc  epsilon
RIa chain, a C' strand hydrogen-bonded to the C strand in D2,
seen in 12 of the 15 structures.
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