The function of RCC1/Ran complex.

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The RCC1 gene was cloned as the human gene complementing tsBN2 cells, a temperature sensitive(ts)mutant of the BHK21 cell line derived from golden hamsters which show the cell cycle defect ; G1 arrest, and premature initiation of mitosi at the restrictive temperature. Its homologes have been isolated from both buddings (SRM1/PRP20/MTR1)and fission yeasts(pim1)and Drosophila(BJ1)in addition to hamster and Xenopus RCC1. Bischoff and Ponstingle have shown that RCC1 function as GNRP(Duanine nucleotide releasing protein)on the nuclear small G protein, Ran/TC4. According to the model of G protein, it pick up a signal from the receptor, in case of a signal transduction from the cell surface, and transfer to the downstream. Thus, some factors corresponding to the receptor should locate upstream of the RCC1/Ran system. The idea that such factor may be DNA molecule is very interesting and apparently reasonable, since RCC1 locates on chromosome abundantly ; one RCC1 molecule per one to ten nucleo … More somes. Our model is the following. The replication state of DNA molecule, such as ; its initiation, elongation and completion, will be transfer through the RCC1 : Ran system to the downstream, including a regulator of MPF, a central cell cycle engine. Based on this model, we suppose that the RCC1 has at least two domains : to bind with the chromosome or with Ran/TC4. These binding domains should locate in the repeat, since the RCC1 protein deleted in the N-terminal side outside of the repeat did complement tsBN2 cells. The identification of such a domain in the RCC1 will be a key step to clarify the function of the RCC1/Ran system on the cell cycle. Thus, we made the RCC1 mutants according to the alanine scanning method. Since in tsBN2 cells, endogeous RCC1 proteins disappear due to the mutation at 39.5ﾟC, we could investigate the functions of mutated RCC1, in vivo, by constructing transformants of tsBN2 cells expressing mutated RCC1 protein. We found that the GNRF activity of mutated RCC1 is correlated with its ability complimentating tsBN2 cells. Less