Expression of proteins controlling transbilayer movement of plasma membrane phospholipids in the B lymphocytes from a patient with Scott syndrome.

Scott syndrome is a rare inherited bleeding disorder in which platelets and other blood cells fail to promote normal assembly of the membrane-stabilized proteases of the plasma coagulation system. The defect in Scott blood cells is known to reflect inability to mobilize phosphatidylserine from inner plasma membrane leaflet to the cell surface in response to an elevation of Ca2+ at the endofacial surface. To gain insight into the molecular basis of this membrane defect, we examined the expression in Scott cells of plasma membrane proteins that have been implicated to participate in the accelerated transbilayer movement of plasma membrane PL. By both reverse transcriptase-polymerase chain reaction (RT-PCR) and functional assay, the level of expression of the multidrug resistance (MDR)1 and MDR3 P-glycoproteins in immortalized B-lymphoblast cell lines from the patient with Scott syndrome were indistinguishable from matched cell lines derived from normal controls. Whereas the plasma membrane of Scott cells are insensitive to activation of the plasma membrane PL scramblase pathway, it had been shown that PL scramblase protein isolated from detergent-solubilized Scott erythrocytes exhibits normal function when incorporated into proteoliposomes (Stout JG, Basse F, Luhm RA, Weiss HJ, Wiedmer T, Sims PJ: J Clin Invest 99:2232, 1997). Consistent with this finding in Scott erythrocytes, we found that Scott lymphoblasts expressed normal levels of PL scramblase mRNA and protein, and that the deduced sequence of PL scramblase in Scott cells is identical to that of normal controls. These data suggest that the defect in Scott syndrome is related either to aberrant posttranslational processing of the PL scramblase polypeptide or to a defect or deficiency in an unknown cofactor that is required for normal expression of plasma membrane PL scramblase function in situ, or alternatively, reflects the presence of a detergent-dissociable inhibitor of this pathway.