CD59, an 18-20-kD complement inhibitor anchored to the membrane via glycosyl phosphatidylinositol (GPI), can induce activation of T cells and neutrophils upon cross-linking with antibody. GPI-anchored molecules cocluster in high mol wt detergent-resistant complexes containing tyrosine kinases that are implicated in the signaling pathway. Exogenous, incorporated GPI-anchored molecules are initially unable to induce activation, presumably because they are not associated with kinases. Here we demonstrate that erythrocyte-derived CD59 incorporated in a CD59-negative cell line acquires signaling capacity in a time-dependent manner. Confocal microscopy revealed an initial diffuse distribution of CD59 that became clustered within 2 h to give a pattern similar to endogenous GPI-anchored molecules. Gel filtration of detergent-solubilized cells immediately after incorporation revealed that CD59 was mainly monomeric, but after 3 h incubation all was in high mol wt complexes and had become associated with protein kinases. Newly incorporated CD59 did not deliver a Ca2+ signal upon cross-linking, but at a time when it had become clustered and associated with kinase activity, cross-linking induced a large calcium transient, indicating that CD59 had incorporated in a specialized microenvironment that allowed it to function fully as a signal-transducing molecule.

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