Killer cell inhibitory receptors (KIR) protect class I HLAs expressing targ
et cells from natural killer (NK) cell-mediated lysis, To understand the mo
lecular basis of this receptor-ligand recognition, we have crystallized the
extracellular ligand-binding domains of KIR2DL2, a member of the Ig superf
amily receptors that recognize HLA-Cw1, 3, 7, and 8 allotypes, The structur
e was determined in two different crystal forms, an orthorhombic P2(1)2(1)2
(1) and a trigonal P3(2)21 space group, to resolutions of 3.0 and 2.9 Angst
rom, respectively. The overall fold of this structure, like KIR2DL1, exhibi
ts K-type Ig topology with cis-proline residues in both domains that define
beta-strand switching, which sets KIR apart from the C2-type hematopoietic
growth hormone receptor fold, The hinge angle of KIR2DL2 is approximately
80 degrees, 14 degrees larger than that observed in KIR2DL1 despite the exi
stence of conserved hydrophobic residues near the hinge region. There is al
so a 5 degrees difference in the observed hinge angles in two crystal forms
of 2DL2. suggesting that the interdomain hinge angle is not fixed. The put
ative ligand-binding site is formed by residues from several variable loops
with charge distribution apparently complementary to that of HLA-C, The pa
cking of the receptors in the orthorhombic crystal form offers an intriguin
g model for receptor aggregation on the cell surface.