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Background  Human importin beta has been used in all  Xenopus laevis in vitro  nuclear assembly and spindle assembly studies. This disconnect between species raised the question for us as to whether importin beta was an authentic negative regulator of cell cycle events, or a dominant negative regulator due to a difference between the human and  Xenopus  importin beta sequences. No  Xenopus  importin beta gene was yet identified at the time of those studies. Thus, we first cloned, identified, and tested the  Xenopus  importin beta gene to address this important mechanistic difference. If human importin beta is an authentic negative regulator then we would expect human and  Xenopus  importin beta to have identical negative regulatory effects on nuclear membrane fusion and pore assembly. If human importin beta acts instead as a dominant negative mutant inhibitor, we should then see no inhibitory effect when we added the  Xenopus  homologue.    Results  We found that  Xenopus  importin beta acts identically to its human counterpart. It negatively regulates both nuclear membrane fusion and pore assembly. Human importin beta inhibition was previously found to be reversible by Ran for mitotic spindle assembly and nuclear membrane fusion, but not nuclear pore assembly. During the present study, we observed that this differing reversibility varied depending on the presence or absence of a tag on importin beta. Indeed, when untagged importin beta, either human or  Xenopus , was used, inhibition of nuclear pore assembly proved to be Ran-reversible.    Conclusion  We conclude that importin beta, human or  Xenopus , is an authentic negative regulator of nuclear assembly and, presumably, spindle assembly. A difference in the Ran sensitivity between tagged and untagged importin beta in pore assembly gives us mechanistic insight into nuclear pore formation.

Xenopus importin beta validates human importin beta as a cell cycle negative regulator