(A) Comparable expression levels of the mutant GPIb-IX complex expressed in CHOαSSβIX cells (dashed line) and the wild type complex in CHOαCCβIX cells (solid line). Surface expression of GPIbα and GPIX were measured by flow cytometry with monoclonal antibodies WM23 and FMC25, respectively. Dark grey peak: CHO cells; light grey peak: CHOβIX cells. (B,C) Conformation of GPIbαSS expressed in CHOαSSβIX cells is largely native-like as the wild type in CHOαCCβIX cells. GPIbα expressed in transfected CHO cells were stained with the indicated conformation-sensitive monoclonal antibodies and measured by flow cytometry. Representative histograms of antibody binding was shown in (B). The mean fluorescence intensities (MFI) were measured from 4–6 independent experiments, normalized to the wild type level and presented as the mean ± SD (C). Groups were compared using the non-paired t test; **, p < 0.01. Binding of mutant GPIbαSS to AK2, another conformation-sensitive antibody, was significantly increased compared to the wild type GPIbαCC, suggesting a local conformational change around GPIbα residues 35– 59, the epitope region recognized by AK2.