ReviewThe nuclear envelope, lamins and nuclear assembly
Introduction
The nuclear envelope (NE) consists of the outer and inner nuclear membranes, nuclear pore complexes (NPCs) and an underlying network of filaments composed of lamin proteins 1., 2.. The outer membrane is part of the rough endoplasmic reticulum (ER), whereas the inner nuclear membrane (INM) has a unique protein composition and is a specialized sub-domain of the ER. Proteins unique to the INM typically bind specifically to lamin filaments and chromatin.
Lamin filaments are polymers of nuclear-specific inter-mediate filament proteins named lamins. There are two types of lamins, A- and B-type, that form stable yet dynamic structures [1]. Vertebrates have three lamin genes: LMNB1 encodes lamin B1; LMNB2 encodes lamins B2 and B3; and LMNA encodes four splice variants comprising lamins A, AΔ10, C1 and C2 [1]. The structures and functions of nuclear lamins and lamin-binding proteins (collectively termed the ‘lamina’) remain central questions. The assembly and function of the lamina are also central to understanding the mechanisms of several inherited diseases (discussed below).
Here, we review advances in this field in the past year. These include the identification of a family of spectrin-repeat-containing INM proteins and other novel inner-membrane proteins, the discovery of a nuclear membrane fusion complex, and growing evidence that A-and B-type lamins and their binding partners have distinct roles during nuclear assembly and interphase.
Section snippets
Plant nuclear assembly in vitro
The mechanism of NE assembly in metazoans has been studied extensively using fractionated, reconstituted extracts from Xenopus and sea urchin eggs, which contain stockpiles of disassembled nuclear components [2]. In a simple yet pioneering study, Lu and Zhai [3••] made cell-free extracts from tobacco plants that can assemble nuclei in vitro, opening the plant kingdom to in vitro reconstitution of NE assembly and function.
Plant nuclear assembly was reconstituted using extracts from Nicotiana
Nuclear pore complex assembly in metazoans
The recruitment of three NPC proteins, Nup153, RanBP2 and Tpr, were compared with the recruitment of INM proteins during NE assembly. Nup153 and RanBP2 are components of the nuclear basket and cytoplasmic fibers of the NPC, respectively [8]. Interestingly, the timing of recruitment of both proteins (around six to eight minutes after the onset of anaphase) coincides with the recruitment of two INM proteins, emerin and the lamin B receptor (LBR) [9]. This is consistent with rapid re-assembly of
Lamins and inner nuclear membrane protein recruitment to the re-forming nuclear envelope
Several INM proteins, including LBR, lamina-associated polypeptide 2β (LAP2β) and emerin, have been studied for possible roles in NE formation. Many of these studies involved chimeric fusions of INM proteins to green fluorescent protein (GFP).
One important new concept is that the assembly of known components, such as lamins and INM proteins, is neither generic nor uniform. Instead, different subsets of proteins are recruited to distinct regions of the re-forming NE during telophase, suggesting
Lamin B associates with A-kinase anchoring proteins and related molecules
Two NE-associated proteins named HA95 (related to A-kinase anchoring protein AKAP95) and AKAP149 may play key roles in recruiting lamin B during NE assembly. Although HA95 is not an integral membrane protein, it is a stable component of the NE and co-immunoprecipitates with lamin B in mammalian cells [18]. Several INM proteins (LBR, LAP2β and emerin) also co-immunoprecipitate with HA95, but these interactions may be mediated indirectly by lamin B [18]. Further interpretation of these findings
Studies with truncated lamins
Gilbert and co-workers [25] tested the in vivo effects of lamin mutations that disrupt lamin assembly or polymerization in vitro. CHO cells transfected with lamin A lacking either the amino-terminal head domain or the carboxy-terminal isoprenylation motif (CAAX) disrupt the structure of endogenous lamins A/C, but have no effect on B-type lamins at the NE. Expression of CAAX-deleted lamin B1 protein also disrupted the NE localization of A-type lamins, but not the localization of endogenous
Nuclear membrane fusion
One holy grail of nuclear assembly, the mechanism of membrane fusion to enclose chromatin, appears to be in hand. Mattaj, Warren and co-workers [27••] found that p97, an AAA-ATPase involved in Golgi and ER fusion events, also mediates nuclear membrane fusion, in two distinct ways. ‘Lateral’ fusion between neighboring NE tubules, mediated by the novel p97–Ufd1–Npl4 complex, allows attached nuclear membranes to enclose the chromatin [27••]. Mechanistically distinct fusion events to enlarge the NE
Lamins, disease and nuclear assembly
Eye-opening discoveries about nuclear lamins came in 1999 and 2000, with reports that mutations in A-type lamins cause a variety of diseases 30•., 31., 32.. Although A-type lamins are expressed in most adult cells, point mutations cause heritable diseases that affect skeletal muscle, heart, tendons or adipose tissue. One such disease is EDMD, which affects skeletal muscle, the cardiac conduction system, and tendons [30•]. The autosomal dominant form of EDMD maps to LMNA, and the X-linked
In vivo studies of lamin function
Loss-of-function studies in C. elegans and Drosophila have shown definitively that B-type lamins are essential for life. C. elegans has a single lamin gene, lmn-1, which is present at the nuclear periphery of all cells except sperm [21]. Notably, C. elegans lamin is also present in the nuclear interior in embryonic cells and some adult cells, consistent with results in mammalian BHK-21 cells showing that the majority of lamin A resides in the nuclear interior [15••]. RNA interference mediated
Inner nuclear membrane proteins
Lamins interact with many different proteins found at the INM, and these attachments are thought to link lamin filaments tightly to the INM. Until recently, only a handful of resident INM proteins were known, including LBR, several isoforms each of the lamina-associated polypeptides (LAPs) 1 and 2, emerin, MAN1, otefin and nurim 38., 39., 40.. Most of these proteins bind directly to either A- or B-type lamins, and also interact directly with chromatin. A large subset of known INM proteins,
Newly identified proteins of the nuclear inner membrane
In a major advance for the field, the number of identified INM proteins almost doubled in the past year. Most of the new INM proteins are novel, including the ring-finger-binding protein (RFBP), luma, nesprins/myne-1 and Unc-50-like (UNCL) (see below), and their potential for binding to lamins has not yet been tested. However, many of these proteins are known to or are predicted to interact with binding partners that will significantly expand the range of known functions for INM proteins.
RFBP
Lamins and gene expression
One significant finding from 2001 was the discovery that LAP2β, the largest membrane-anchored isoform of LAP2 in mammals, can transcriptionally repress E2F–DP-regulated genes either alone, or additively with a ubiquitous transcriptional repressor named ‘germ-cell-less’ (GCL) [59••]. GCL co-localizes with LAP2β at the NE, and binds directly to LAP2 in vitro [59••]. GCL is the fourth domain-specific binding partner discovered for LAP2β, in addition to lamin B, BAF and DNA. GCL represses
Conclusions
This field is now progressing rapidly, particularly in terms of identifying nuclear membrane proteins and characterizing their attachments to lamins and other important binding partners. As these links become clearer, the functions of lamins and their binding partners will surely emerge. It will be important to test all INM proteins for possible roles in nuclear assembly, even if their currently known partners might suggest interphase-restricted roles. If anything, the history of this field
Update
Four recently published papers are noteworthy. Two groups report that NE breakdown is stimulated by the attachment of spindle microtubules to the NE 61., 62.. Force generated by the spindle pulls the NE towards each spindle pole [61]. Both dynein (a microtubule motor protein) and dynactin (which regulates dynein) concentrate at the NE, and are thought to provide the pulling force [62].
The other two papers relate to lamins and disease 63., 64.. Spann and colleagues [63] directly tested whether
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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