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  2. Three Brief Papers on Nuclear Pore Complexes

Three Brief Papers on Nuclear Pore Complexes

By apalazzo on May 31, 2006.

The nuclear membrane separates the nuclear space from the cytoplasm. This barrier is comprised of two membranes (Inner and Outer Nuclear Membrane) that are continuous with the endoplasmic reticulum. To cross the double membrane, molecules traverse the nuclear pore complex (NPC), a giant macromolecular complex that has an eight-fold symmetry and weighs over 100MDa. To date, only two components of the NPC have been identified: gp210 and Pom120. Interestingly many cells only express one of these proteins. Well it seems like Dirk Gorlich's group have been able to knock out BOTH genes from Hela cells ... and the cells are fine! They have NPCs, they can reform the nuclear membrane after mitosis ... something strange is going on (On top of that another group previously claimed that gp210 was essential ... in Hela cells!)

Nuclear pore complex assembly and maintenance in POM121- and gp210-deficient cells
Fabrizia Stavru, Gitte Nautrup-Pedersen, Volker C. Cordes, and Dirk Görlich
JCB (2006) 173:477-83

In a second paper the Gorlich group solve the problem ... there is a third integral membrane protein that is part of the NPC, called Ncd1. Turns out that yeast don't have gp210 or Pom120 orthologues. Instead they have Ncd1 and two other non-conserved integral membrane proteins, Pom152 and Pom34. Of the three, Ncd1 is the only protein that is essential and conserved in other organisms. Interestingly Ncd1 is also part of the yeast spindle pole body, a structure related to our centrosomes ... (Does this sound interesting Gomez?). The Gorlich lab show that there are Ncd1 orthologues that localize to NPCs in fly, frogs, worms, humans

The Gorlich lab knocked down Ncd1 in Hela and the NPCs were defective (lower staining of NPC components). Worm knockouts were mostly non-viable.

NDC1: a crucial membrane-integral nucleoporin of metazoan nuclear pore complexes Fabrizia Stavru, Bastian B. Hülsmann, Anne Spang, Enno Hartmann, Volker C. Cordes, and Dirk Görlich JCB (2006) 173:509-19

i-317ad31b813bd92883548949a8292498-npc1.jpg

Last is a paper from Martin Hetzer's lab. Here they tackle the tricky question of how do you form NPCs. There are two times when you have to form these structures; after mitosis when the nuclear envelope reforms, and during interphase as the nucleus expands. Using complicated in vitro manipulations of nuclei formed from frog egg extracts, the Hertzer lab shows that active Ran is required both in the nuclear and cytoplasmic compartments to activate NPC formation in an expanding nucleus (an analogous situation to NPC formation in interphase). They offer evidence that ran acts to dissociate the Nup107complex, which is a major constituent of the NPC) from Importin-beta in both the nucleoplasm and cytoplasm. Interestingly they show that excess free Nup107 complex can stimulate NPC formation IN NUCLEI THAT DO NOT HAVE NPCs. That means that exogenously added Nup107 complex can stimulate the fusion of the Outer Nuclear Membrane and Inner Nuclear Membrane, to form a hole where the NPC sits. They then observe that gp210 form patches on the nuclear envelope and may act as landing pads for Nup107 complex in intact nuclei.

So it is now clear that within the lumen of the nuclear envelope there's a machine that can fuse the inner and outer nuclear membranes inorder to form pores.

What is the machine? Well it looks like of the integral membrane proteins, gp210 and Pom120 are dispensable for NPC formation, so scratch those off the list. Ncd1? There are survivors of the Ncd1 knockouts in worms. These survivors also have NPCs. Also Ncd1 doesn't have a big luminal domain. Thus it is likely that a whole fusion complex exists in the nuclear lumen ... waiting for someone to find it ...

Nuclear Pores Form de Novo from Both Sides of the Nuclear Envelope
Maximiliano A. D'Angelo, Daniel J. Anderson, Erin Richard, Martin W. Hetzer
Science (2006) 312:440-443

Cross posted at Science Sampler.

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RPM,

Very cool. I guess that since the NPC is very intricate it is easy to mess up through the expression of incompatible alleles.

On a (not too) related note, the Ncd1 paper discusses the conservation of the three integral membrane components of the NPC. None are 100% conserved, yet two are very ancient and probably part of the ancestral NPC. Since both unikont and bikont nuclear membranes fall apart and reassemble after mitosis (with some exceptions) there must be some conserved (yet undiscovered) machinery that fuses the outer and inner nuclear membrane. From Stavru et al.,

Consistent with the concept of redundancy and robustness, NPCs appear not to rely on just a single anchor within the NE. Instead, they typically contain several membrane-integral Nups (e.g., three different ones in either yeast or mammals). Genomic data indicate that two of them, gp210 and NDC1, are evolutionary conserved (Mans et al., 2004). The fact that both are found in metazoans as well as in plants, clearly suggests that they evolved before the unikont/bikont bifurcation, which is considered as the oldest time point of a major evolutionary diversifi cation of known eukaryotes (Richards and Cavalier-Smith, 2005). Primordial NPCs were therefore probably equipped with both gp210 and NDC1. However, it appears that some lineages (e.g., all fungi) lost gp210, whereas other lineages (e.g., Dictyostelium discoideum or other protozoa) lost NDC1 from their genomes. This brings us to the unexpected conclusion that none of the integral Nups is?generally and in all cellular settings?essential for NPC assembly and function. This also explains why the nematode C. elegans
can live, although miserably, in the absence of NDC1, why many mammalian cell types, such as fi broblasts, assemble fully functional NPCs without gp210, and why POM121 can be depleted from human cells without deleterious defects.