THE ROLE OF THE NUCLEAR ENVELOPE IN INTRACELLULAR PROTEIN SORTING

通过核孔复合物的运输是必不可少的调节

Transport across the nuclear pore complex is essential for regulating cell growth and normal development. The structure of the nuclear pore and its involvement in nuclear transport are being studied at a molecular level. The nuclear pore complex is made up of a family of phosphorylated glycoproteins having covalently attached O-linked N- acetylglucosamine. We have expressed the major nuclear pore glycoprotein p62 in bacteria and examined the glycosylation in vitro. Glycosylated p62 is also obtained by expression in insect cells using baculovirus. The sites of glycosylation of p62 have been determined by deletion analysis and protease mapping. The enzyme responsible for glycosylation of the nuclear pore glycoproteins has been partially purified from rabbit reticulocyte lysate; an attempt is being made to molecularly cloning this enzyme. To examine the function of the nuclear pore glycoproteins in vitro, transport, Xenopus laevis extracts capable of nuclear assembly and transport have been employed. Using this system, it was demonstrated that the O-linked N-acetylglucosamine moiety of nuclear pore glyco- proteins can be modified without altering nuclear transport. These extracts have also been used to demonstrate that the glycosylation and phosphorylation of nuclear pore glycoproteins change during the cell cycle. The findings suggest that there may be a coordinate regulation of glycosylation and phosphorylation which accompany the breakdown and reassembly of the nuclear pore during the cell cycle.