THE AMINO-TERMINUS OF MAMMALIAN NUCLEOLIN SPECIFICALLY RECOGNIZES SV40 T-ANTIGEN TYPE NUCLEAR-LOCALIZATION SEQUENCES

Nucleolin is a major nucleolar protein in mammalian cells that is thou ght to be involved in ribosome biogenesis. The discovery that nucleoli n shuttles between the cytoplasm and the nucleus raises the possibilit y that it is also involved in transporting ribosomal or nuclear protei ns to the nucleus. The three structural domains of nucleolin bear a st riking resemblance to the domains of a previously identified yeast pro tein NSR1, although the two proteins do not share a high degree of seq uence similarity. NSR1 specifically recognizes the nuclear localizatio n sequence (NLS) of both the simian virus large T antigen (SV40 T-anti gen) and the yeast histone H2B by ligand blot analysis, and is a candi date for a receptor involved in the initial stages of nuclear transpor t. We report here that nucleolin, either purified from Chinese hamster ovary (CHO) cells or expressed in yeast, also specifically recognizes the wild-type, but not a mutant, histone H2B nuclear localization seq uence by ligand blot analysis. The NLS recognition site is located wit hin the N-terminal domain of both proteins. In showing that nucleolin, a protein that moves between the cytoplasm and the nucleus, also has the ability to interact with nuclear localization signals, our data su pport the idea that shuttling nucleolar proteins play a role in nuclea r transport.