Assembling pores

Despite the prominence of the nuclear pore structure, its assembly mechanism remains largely undescribed. On page 361, Madrid et al. show that one of the three transmembrane pore proteins is required for interphase assembly in yeast. The data suggest that insertion of nascent pores into the interphase nuclear envelope proceeds in a manner similar to that of postmitotic pore formation.

In budding yeast lacking Pom34 or Pom152, two of the transmembrane proteins, soluble pore components localized normally to the nuclear envelope. However, nuclear pore components were significantly mislocalized in cells depleted only for the third transmembrane protein, Ndc1. The Ndc1 defect was exaggerated in cells lacking Pom152.

Electron microscopy showed that double mutants had wide openings in the nuclear envelope, but little associated protein. These gaps allowed indiscriminant movement through the envelope, with the occasional mislocalized ribosome showing up in the nucleus.

Comparison of these results to previously published work from Xenopus suggests that the assembly of both interphase and postmitotic pores rely on the initial localization of these transmembrane proteins. Significantly, Mansfeld et al. recently identified the mammalian homologue of Ndc1 and found that it too was essential for pore assembly (Mol. Cell. 2006. 22:92–103).

Madrid et al. are now working on the development of in vivo assays to monitor the formation of new pores and learn just how Ndc1 controls the process.