An essential step in retrovirus infection is the binding of the virus to its receptor on a target cell. The structure of the receptor-binding domain of the envelope glycoprotein from Friend murine leukemia virus was determined to 2.0 angstrom resolution by x-ray crystallography. The core of the domain is an antiparallel β sandwich, with two interstrand loops forming a helical subdomain atop the sandwich. The residues in the helical region, but not in the B sandwich, are highly variable among mammalian C-type retroviruses with distinct tropisms, indicating that the helical subdomain determines the receptor specificity of the virus.
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