From nuclear pore to kinetochore

Nuclear proteins are turning up in some odd places lately. First, the nuclear-transport factor Ran was implicated in spindle formation. Then, the mitotic checkpoint proteins Mad1 and Mad2 turned up at the nuclear pore, in a switch with the mRNA export factor hRae1, which appeared with the mitotic checkpoint protein mBUB1 at the kinetochore. Now, Belgareh et al. report that two structural nuclear pore complex constituents also localize to the kinetochore (page 1147).

Belgareh et al. were interested in characterizing the human version of a budding yeast nuclear pore subcomplex that is involved in mRNA export. They identified hNup133 by homology, and identified three other members of the subcomplex through immunoprecipitation experiments.

The surprise came in the localization studies, when both antibody staining and GFP fluorescence indicated that at least two of these nucleoporins are found at kinetochores from prophase through to late anaphase.

This simple observation has many possible interpretations, none of which (as yet) comes with any significant supporting evidence. First, nucleoporins at the kinetochore could act to seed the formation of a specific subset of nuclear pores at the end of mitosis, while the nuclear envelope reforms around chromosomes. Second, the nucleoporins could have some unspecified function at the kinetochore, although a role in spindle formation is not obvious as none of the nucleoporins under discussion has the FG repeats characteristic of proteins that link indirectly to Ran.

Finally, the nucleoporins may visit the kinetochore merely to pick up mitotic checkpoint proteins, so that those checkpoint proteins can be sequestered at the nuclear pores during the following cell cycle. Why that might be necessary is anyone's guess. ▪