Intracellular redistribution of interferon-inducible proteins Nmi and IFP 35 in apoptotic cells

Abstract

As a consequence of the activation of the apoptotic pathway, the subcellular localization of many proteins that modulate cell death is altered. Nmi and IFP 35 are homologous interferon (IFN)-inducible proteins that associate into a 300-400-kDa complex in the cytoplasm. Prior to treatment with IFN, subcellular fractionation reveals that the proteins are found primarily in a soluble cytoplasmic fraction, and immunofluorescence shows a diffuse cytoplasmic distribution. Treatment with IFN results in some Nmi and IFP 35 fractionating with membranes and colocalizing into speckles as detected by immunofluorescence. In IFN-treated apoptotic cells, the speckles dissociate and are replaced by a diffuse cytoplasmic distribution of Nmi and IFP 35. In Jurkat cells, dissociation of speckles could not be separated from other markers of apoptosis, including exposure of phosphatidylserine on the cell surface, caspase cleavage of D4-GDI, and nuclear condensation and fragmentation. The caspase inhibitor, zVAD-fmk, was not able to inhibit the dissociation of speckles in vivo or in a permeabilized cell assay in vitro, suggesting that dissociation of speckles is a caspase-independent process. Nmi and IFP 35 proteins are not caspase substrates, as total levels of Nmi and IFP 35 and their association into a cytoplasmic complex do not change in apoptotic cells. Consistent with dissociation of speckles, subcellular fractionation studies show redistribution of Nmi and IFP 35 from the membrane fraction into the S100 fraction of apoptotic cells. Our studies have revealed dissociation of the IFN-induced Nmi/IFP 35 speckles as a newly defined specific event occurring during apoptosis.