The morphology of organelles is intimately related to their function and deviation from normal morphology can have profound physiological consequences. It is thus plausible that NE shape alterations contribute to events which cause cellular malfunction in disease.
Nevertheless, radical changes of the composition of the nuclear periphery can be compatible with cell survival, as in a variety of yeast deletion strains lacking transmembrane proteins which localize to the NE and ER (Nem1p, Spo7p, Ssh1p), nucleoplasmic proteins (Thp1p), or nucleoporins (Seh1p) [14
]. None of these proteins normally are restricted to the periphery of the nucleoplasm itself.
Interestingly, the shape of the nucleus is not affected by deletion of Esc1p or deletion of two other proteins which normally localize to the periphery of the nucleoplasm, Mlp1p and Mlp2p [9
]. Moreover, the growth of nem1-Δ, seh1-Δ, spo7-Δ, ssh1-Δ
] is not obviously affected by overexpression of Esc1p (not shown).
Mutations in nuclear lamina constituents, most notably lamin A and C, cause a diverse spectrum of diseases, the laminopathies. Laminopathies caused by excess pre-lamin A at the nuclear periphery are characterized by bleb-like expansions of the nuclear surface [41
]. The dependence of the shape of the yeast nucleus on both nuclear membrane proteins and proteins that concentrate at the periphery of the nucleoplasm is reminiscent of a distinct laminopathy (Emery-Dreyfuss Muscular Dystrophy), which can result from either mutation of the inner nuclear membrane protein, emerin, or mutation of lamin A [45
]. Moreover, overexpression of a GFP-tagged form of the inner membrane protein, Prm3p, distorts the shape of the NE in much the same fashion as Esc1p, and excess Heh1p and Heh2p [24
] grossly distort the NE and chromatin mass, while Mlp1p induction has no obvious impact. These differential effects may signify that the Heh proteins have a high affinity for chromatin, that Esc1p and Prm3p are more closely linked to the inner nuclear membrane per se
than to chromatin, and that Mlp1p is relatively independent.
The characteristic structure of escapades and distribution of excess GFP-Esc1p are compatible with the hypothesis that excess Esc1p forces enlargement of the NE due to end-to-end association of the protein, that chromatin has an intrinsic coherence which tends to preserve a roughly globular shape, and that the inner aspect of the NE (or perhaps Esc1p itself) can also self-associate laterally (Fig. ). Indeed, although there has never been an experimentally accessible model for investigation of this latter issue, the NE of malignant cells – like escapades – is frequently characterized by focal self-apposition of the lamina and/or the nucleoplasmic surface of the inner nuclear membrane [47
Figure 8 Model of Escapade Structure. Nuclear pore complexes (NPC), outer nuclear membrane (Outer), inner nuclear membrane (Inner), and Esc1p are indicated. The small circles at the surface of the membranes are ribosomes. The model has been drawn to illustrate (more ...)
There is no reason to expect that Esc1p is fully comparable to higher eukaryotic lamins. For example, unlike lamins, tagged Esc1p (or Mlp1p) expressed from its own promoter does not completely encircle the nucleus, being absent from beneath the nucleolus [13
]. Moreover, the observation of rapid diffusion of GFP-Esc1p upon karyogamy shows that at least the overexpressed protein is mobile.
The transcriptional consequences of overexpressing Esc1p emphasize the importance of this protein (or escapades themselves) for gene expression. Since both negative and positive changes are seen, Esc1p appears to be a complex regulator, not only an enhancer of silencing. In this regard, it resembles many transcriptional regulators including Rap1p [49
]. It is also notable that ~70% of the changes which we detect occur at loci which are further than 40 kb from telomeres (which concentrate at the periphery). In addition to their intrinsic interest, these microarray data provide a possible prototype against which to evaluate the transcriptional characteristics of laminopathies, which presumably account for their cell type-specific effects.
Daughter cells differ from mothers in several regards [50
]. Escapades are not transferred to daughter cells, apparently due to their immobility. These structures – and karmellae – are thus part of a "lagging" domain of the nuclear perimeter. The nucleolus may also be part of this domain, judging from its association with escapades and karmellae, as well as the observation that it is one of the last nuclear components to reach the bud during anaphase [52
]. Extra rDNA circles [54
] and ARS plasmids [55
] are also retained.