Monoclonal antibodies to a Mr 68,000 pore complex glycoprotein interfere with nuclear protein uptake in Xenopus oocytes.

Using a monoclonal antibody (PI1) raised against mouse lymphocyte nuclear matrix fractions we have identified a N-acetylglucosamine (GlcNAc)-containing glycoprotein of Mr 68,000 as a component of the nuclear pore complexes of Xenopus laevis oocytes. The antigenic determinant recognized by antibody PI1 comprises both the sugar moiety and protein sequences since, on the one hand, added GlcNAc competed effectively for antibody binding and, on the other hand, the antibody reacted in immunoblots with only one member of the GlcNAc-containing pore complex glycoprotein family. By using immunogold-electron microscopy we could demonstrate that the Mr 68,000 glycoprotein was located preferentially to the cytoplasmic side of the pore complex channel. When radiolabeled soluble nuclear proteins were injected into the cytoplasm of Xenopus oocytes, their reentry into the nucleus was almost completely inhibited in the presence of antibody PI1 as shown by two-dimensional gel electrophoresis. The results indicate that the evolutionarily conserved Mr 68,000 glycoprotein is involved in transport processes of karyophilic proteins from the cytoplasm into the nucleus.