Daigle et al. (page 71; see also the Comment on page 17) have found that NPCs are remarkably stable complexes that appear to be anchored to a protein network in the nuclear envelope. The work also demonstrates the feasibility of tracking single protein complexes in living cells.
By fusing GFP tags to POM121, lamin B1, or Nup153, the authors were able to track the movement and turnover of NPCs directly. Instead of diffusing freely within the plane of the nuclear envelope during interphase, NPCs remained largely immobile, only moving slowly in unison in large arrays. Individual subunits of the NPC vary considerably in their turnover rates, with POM121 turning over less than once per cell cycle, whereas Nup153 turns over three orders of magnitude faster. The slow turnover of POM121 indicates that the NPC remains stable throughout interphase. This stability, the authors suggest, allows NPCs to form part of a crosslinked protein network in the nuclear envelope composed of lamins, inner nuclear membrane proteins, and peripheral heterochromatin. ▪