Importin-β and the small guanosine triphosphatase Ran mediate chromosome loading of the human chromokinesin Kid

Nucleocytoplasmic transport factors mediate various cellular processes, including nuclear transport, spindle assembly, and nuclear envelope/pore formation. In this paper, we identify the chromokinesin human kinesin-like DNA binding protein (hKid) as an import cargo of the importin-α/β transport pathway and determine its nuclear localization signals (NLSs). Upon the loss of its functional NLSs, hKid exhibited reduced interactions with the mitotic chromosomes of living cells. In digitonin-permeabilized mitotic cells, hKid was bound only to the spindle and not to the chromosomes themselves. Surprisingly, hKid bound to importin-α/β was efficiently targeted to mitotic chromosomes. The addition of Ran–guanosine diphosphate and an energy source, which generates Ran–guanosine triphosphate (GTP) locally at mitotic chromosomes, enhanced the importin-β–mediated chromosome loading of hKid. Our results indicate that the association of importin-β and -α with hKid triggers the initial targeting of hKid to mitotic chromosomes and that local Ran-GTP–mediated cargo release promotes the accumulation of hKid on chromosomes. Thus, this study demonstrates a novel nucleocytoplasmic transport factor–mediated mechanism for targeting proteins to mitotic chromosomes

Enhancement of Zidovudine Uptake by Dehydroepiandrosterone Sulfate in Rat Syncytiotrophoblast Cell Line TR-TBT 18d-1

ABSTRACT: AZT (3-azido-3-deoxythymidine; zidovudine), which is used for the prevention of mother-to-child transmission of HIV-1, is transplacentally transferred to the fetus across the blood-placenta barrier, which is composed of syncytiotrophoblasts. We recently showed that apical uptake of AZT by syncytiotrophoblasts is mediated by saturable transport system(s) in the TR-TBT 18d-1 cell line, and the cellular accumulation of AZT was increased in the presence of dehydroepiandrosterone sulfate (DHEAS). Here, we aimed to clarify the mechanism of this effect of DHEAS. Inhibitors of efflux transporters, including breast cancer resistance protein, P-glycoprotein, and multidrug resistance proteins, had little effect on the cellular accumulation of AZT in TR-TBT 18d-1. Kinetic study revealed that the rate constant for AZT uptake was greatly increased in the presence of 1 mM DHEAS. These results suggested that the effect of DHEAS was because of enhancement of the uptake process(es), rather than inhibition of efflux. When AZT uptake was analyzed according to the Michaelis-Menten equation, the estimated Michaelis constant, K m , for AZT uptake in the presence of 1 mM DHEAS was lower than that in its absence, whereas maximum uptake velocity, V max , and nonsaturable uptake clearance, k ns , were similar in the presence and absence of DHEAS, indicating that DHEAS may change the recognition characteristics of the transporter for AZT in TR-TBT 18d-1. Thus, the increase of AZT uptake in TR-TBT 18d-1 cells in the presence of DHEAS was concluded to be because of a DHEAS-induced change in the affinity of AZT uptake system, although the transporter responsible for AZT uptake has not been identified

Differential Expression of Ezrin and CLP36 in the Two Layers of Syncytiotrophoblast in Rats

The syncytiotrophoblast, which regulates maternal-fetal transfer of drugs, consists of a single layer in humans, but two layers, i.e., SynI and SynII, in rodents. Polar distribution of transporters in the apical and basal plasma membranes of syncytiotrophoblast is important for placental function in terms of vectorial transport of substrates, but the mechanisms that control protein distribution in the syncytiotrophoblast remain unclear. We have previously established rat syncytiotrophoblast cell lines, TR-TBT 18d-1 and TR-TBT 18d-2, which retain characteristics of SynI and SynII, respectively. In this study, we aimed to characterize the gene expression profiles in the two layers by using these cell lines. DNA microarray analysis indicated that more than 25 mRNAs, including cytoskeleton binding proteins, ezrin and CLP36, are differentially expressed between TR-TBT 18d-1 and TR-TBT 18d-2. Quantitative real time-polymerase chain reaction (PCR) analysis indicated that mRNA expression of ezrin, CLP36, CCN1, and CCN2 is higher in TR-TBT 18d-1 and mRNA expression of elf-1a, hsc70 and flot2 is higher in TR-TBT 18d-2, compared with their counterparts. Immunohistochemical analysis indicated that ezrin is expressed in rat placental villi in vivo, and is located on the apical membranes of TR-TBT 18d-1, while CLP36 is located in the apical and basal sides of TR-TBT 18d-1. The expression of ezrin was highest at gestational days 14 and 18 and was highest among the ezrin/radixin/moesin (ERM) family members. These results may help to clarify the mechanisms controlling polarization of the syncytiotrophoblast and the significance of the double epithelial layers in rat and mouse