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Fc receptors expressed in the gut of newborn rodents bind to maternal
immunoglobulin in milk at pH 6.5, and transport it to the bloodstream of the
neonate, where it dissociates at pH 7.4. The rat intestinal Fc receptor (FcRn)
consists of a heavy chain, with significant sequence similarity to the heavy
chain of class I MHC molecules, complexed to the class I light chain, beta
2-microglobulin. Although FcRn is predicted to contain a groove analogous to
that which serves as the MHC peptide-binding site, the immunoglobulin ligand of
FcRn is a macromolecule instead of a peptide. We have expressed and crystallized
a secreted form of FcRn, and here report the crystallization of a complex
between FcRn and its Fc ligand. Isolated FcRn-Fc complexes crystallize in space
group I222 or I2(1)2(1)2(1) with unit cell dimensions a = 125 A, b = 152 A and c
= 216 A. The crystals diffract to 5.5 A resolution with anisotropic diffraction
to 3.5 A. Data collection from cryopreserved crystals may allow the resolution
limit to be extended, since the major reason for the poor resolution appears to
be radiation decay. Even a low-resolution view of how FcRn binds Fc would be of
interest to see if the binding site corresponds to the functional part of an MHC
molecule. Since the structure of Fc is known, and a structure determination of
FcRn is underway, it may be possible to locate the Fc binding site on FcRn at
low resolution. As an initial characterization of the FcRn-Fc mode of
interaction, and to facilitate the structure determination, we have determined
the stoichiometry of binding of FcRn to Fc. We show that two FcRn molecules bind
per Fc, as determined by analysis of gels of washed crystals, a column binding
assay, and isothermal titration calorimetry.
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