Nuclear relocalization of the pre-mRNA splicing factor PSF during apoptosis involves hyperphosphorylation, masking of antigenic epitopes, and changesin protein interactions

The spatial nuclear organization of regulatory proteins often reflects thei r functional state. PSF, a factor essential for pre-mRNA splicing, is visua lized by the B92 mAb as discrete nuclear foci, which disappeared during apo ptosis. Because this mode of cell death entails protein degradation, it was considered that PSF, which like other splicing factors is sensitive to pro teolysis, might be degraded. Nonetheless, during the apoptotic process, PSF remained intact and was N-terminally hyperphosphorylated on serine and thr eonine residues. Retarded gel migration profiles suggested differential pho sphorylation of the molecule in mitosis vs. apoptosis and under-phosphoryla tion during blockage of cells at G1/S. Experiments with the use of recombin ant GFP-tagged PSF provided evidence that in the course of apoptosis the an tigenic epitopes of PSF are masked and that PSF reorganizes into globular n uclear structures. In apoptotic cells, PSF dissociated from PTB and bound n ew partners, including the U1-70K and SR proteins and therefore may acquire new functions.