Combined pseudo-merohedral twinning, non-crystallographic symmetry and pseudo-translation in a monoclinic crystal form of the γδ T-cell ligand T10

T10 is a non-classical class Ib-like major histocompatibility complex (MHC) cell-surface antigen which binds directly to certain γδ T-cell receptors in the absence of any exogenous and endogenous ligands, such as peculiar lipids or glycolipids. The crystal structure at 2.5 Å resolution of murine T10 was determined by molecular replacement using data from an almost perfectly twinned monoclinic crystal. The space group is P2₁, with unit-cell parameters a = 78.2, b = 70.0, c = 139.2 Å, β = 106.8°. Self-rotation function analysis and various intensity statistics revealed the presence of pseudo-merohedral twinning, but these tests underestimated the true twin fraction of α ≃ 0.46. Native Patterson analyses pointed to the presence of pseudo-translation among the four molecules present in the asymmetric unit. Data analysis, structure determination and model refinement are discussed.