Nucleoplasmin

Abstract: Selective transport of proteins is a major mechanism by which biochemical differences are maintained between the cytoplasm and nucleus. To begin to investigate the molecular mechanism of nuclear transport, we used an in vitro transport system composed of a Xenopus egg extract, rat liver nuclei, and a fluorescently labeled nuclear protein, nucleoplasmin. With this system, we screened for inhibitors of transport. We found that the lectin, wheat germ agglutinin (WGA), completely inhibits the nuclear transport of fluorescently labeled nucleoplasmin. No other lectin tested affected nuclear transport. The inhibition by WGA was not seen when N-acetylglucosamine was present and was reversible by subsequent addition of sugar. When rat liver nuclei that had been incubated with ferritin-labeled WGA were examined by electron microscopy, multiple molecules of WGA were found bound to the cytoplasmic face of each nuclear pore. Gel electrophoresis and nitrocellulose transfer identified one major and several minor nuclear protein bands as binding 125I-labeled WGA. The most abundant protein of these, a 63-65-kD glycoprotein, is a candidate for the inhibitory site of action of WGA on nuclear protein transport. WGA is the first identified inhibitor of nuclear protein transport and interacts directly with the nuclear pore.

Abstract: Molecular chaperones are a ubiquitous family of proteins whose proposed role is to mediate the folding and assembly of other proteins into oligomeric structures. The essential function of molecular chaperones is to prevent the formation of incorrect structures which may result from the transient exposure of charged or hydrophobic surfaces normally involved in interactions between or within polypeptide chains. Such transient exposure may occur during the synthesis of polypeptides, the unfolding and refolding that occurs during their transport across membranes, the association of polypeptides made in one subcellular compartment with those made in another, changes in protein-protein interactions during the normal functioning of a complex, and recovery from stresses such as heat shock. Three classes of molecular chaperone are discussed: the nucleoplasmins, the BiP group, and the chaperonins.