Sugar-dependent nuclear import of glycoconjugates from the cytosol.

Large proteins (above 40 kDa) do not migrate from the cytosol to the nucleus except when they contain or are substituted with specific peptides called nuclear localization sequence. Accordingly fluorescein-labeled serum albumin, introduced into the cytosol upon electroporation, does not leave the cytosol. Conversely the same protein substituted with about 25 sugar residues migrate from the cytosol to the nucleus in a manner dependent on the nature of the sugar linked to serum albumin, on the temperature, and on the incubation time. Serum albumin substituted with glucose, fucose, or mannose residues enter the nucleus at 37 degrees C within 30 min while sugar-free serum albumin or serum albumin substituted with galactoside or 6-phosphomannoside residues do not. At 4 degrees C, all of those proteins stay in the cytosol. Thus, glycosylated proteins can enter the nucleus from the cytosol by making use of the sugars borne either by a new type of nuclear import or by making a complex with lectins acting as shuttle between cytosol and nucleus.