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 gammadelta T-cell receptors in the absence of any exogenous and endogenous ligands, such as peculiar lipids or glycolipids. The crystal structure at 2.5 Angstrom resolution of murine T10 was determined by molecular replacement using data from an almost perfectly twinned monoclinic crystal. The space group is P2(1), with unit-cell parameters a=78.2, b=70.0, c=139.2 Angstrom, beta=106.8degrees. Self-rotation function analysis and various intensity statistics revealed the presence of pseudo-merohedral twinning, but these tests underestimated the true twin fraction of alphasimilar or equal to0.46. 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.