Nuclear lamins: major factors in the structural organization and function of the nucleus and chromatin

Abstract

Over the past few years it has become evident that the intermediate filament proteins, the types A and B nuclear lamins, not only provide a structural framework for the nucleus, but are also essential for many aspects of normal nuclear function. Insights into lamin-related functions have been derived from studies of the remarkably large number of disease-causing mutations in the human lamin A gene. This review provides an up-to-date overview of the functions of nuclear lamins, emphasizing their roles in epigenetics, chromatin organization, DNA replication, transcription, and DNA repair. In addition, we discuss recent evidence supporting the importance of lamins in viral infections.

Figure 1.

Structure of nuclear lamins. (A) Schematic drawing of a pre-lamin polypeptide chain. The central α-helical rod domain (red), the NLS (gray), the Ig-fold (blue; its simplified structure indicating the nine β-sheets is depicted), and the C-terminal −CAAX box (green) are shown. (B) Post-translational processing of pre-lamin A, B1, and B2: A farnesyl group is attached to the cystein residue of the −CAAX box by a farnesyltransferase; the last three residues (−AAX) are proteolytically cleaved off by an AAX endopeptidase; the carboxylic acid group (−COOH) of the C-terminal cysteine residue is methylated by a carboxyl methyltransferase. These steps lead to mature lamin B1 and B2. In the case of farnesylated/carboxymethylated pre-lamin A, an additional 15 C-terminal residues, including the farnesylated/carboxymethylated cysteine, are cleaved off by Zmpste24/FACE1. Inhibition of this step either due to mutations or deficiency in Zmpste24/FACE1 (indicated by the red X) or by protease inhibitors (PIs) can lead to severe phenotypes in mice and humans (for details, see the text). Note that inhibition of farnesylation by farnesyltransferase inhibitors (FTIs) results in a complete loss of processing. (C) Schematic drawings of the C-terminal tail domains of mature lamins A, C, B1, and B2. The following amino acid positions are indicated: start of the tail domain, first residue of the NLS, start and end of the Ig-fold, and C-terminal residue of the respective mature lamin. Note that lamins B1 and B2 are farnesylated and carboxymethylated, while lamins A and C are not.

Figure 2.

Comparison of lamin A, lamin C, and LAΔ50 structures. (A) The 12 exons encoding lamin A and the residues 1–566 of lamin C (exons 1–10) are indicated along with their corresponding amino acid residues. In addition, the positions of 18 mutations associated with Hutchinson-Gilford progeria syndrome (HGPS) are shown (the number of incidents reported for each mutation is shown in parentheses) (http://www.umd.be:2000; http://www.dmd.nl): (blue diamond) 29 C > T (1); (dark-blue diamond) 412 G > A (1); (yellow diamond) 428 C > T (1); (green diamond) 433 G > A (1); (purple diamond) 899 G > A (1); (orange diamond) 1411 C > T + 1579 C > T (1); (dark-blue diamond) 1454 C > G (1); (light-blue diamond) 1583 C > T + 1619 C > T (1); (light-green diamond) 1626 G > C (4); (dark-purple diamond) 1733 A > T (1); (blue star) 1821 G > A (1); (red star) 1822 G > A (1); (purple star) 1824 C > T (37); (pink diamond) 1868 C > G (2); (dark-green diamond) 1930 C > T (1); (brown star) 1968 + 1 G > A (1). Mutations marked with a star lead to abnormal splicing of the last 150 nucleotides of exon 11, resulting in the expression of a mutant lamin A protein (pre-LAΔ50) lacking 50 amino acids from the C terminus of pre-lamin A (residues 607–656). These residues contain the second proteolytic cleavage site (magenta; residue 646) involved in pre-lamin A processing. For a description of the schematic drawing of pre-lamin A, pre-LAΔ50, and lamin C, see Figure 1, A and C. The six lamin C-specific residues (567–572) are indicated (light blue). Note that while most mutations affect both lamin A and C, some are specific for lamin A. (B) Differences in the post-translational processing between pre-lamin A and pre-LAΔ50. The amino acid sequences of pre-lamin A (residues 603–664) and pre-LAΔ50 (residues 603–614) are shown. Residues 607–656 in pre-lamin A, which are absent in pre-LAΔ50, are labeled blue, and the Zmpste24/FACE1 cleavage site (YL) within this region is labeled magenta. The first three steps, including farnesylation and carboxymethylation of the cystein residue of the −CAAX box (−CSIM), are identical for both proteins (for details, see Fig. 1B). In the last step, the 15 C-terminal residues—including the farnesylated/carboxymethylated cysteine residue—are cleaved off from pre-lamin A by Zmpste24/FACE1, resulting in mature lamin A (residues 1–646) and a 15-amino-acid-long farnesylated/carboxymethylated peptide. As the Zmpste24/FACE1 cleavage site is missing in the mutant protein, LAΔ50 remains farnesylated and carboxymethylated.

Figure 3.

Electron microscopic observation of normal and HGPS fibroblasts. Low-magnification views show peripheral heterochromatin and nucleoplasmic heterochromatic foci in the normal nucleus (A), which are both absent in the highly lobulated HGPS nucleus (B). A high-magnification view of the nuclear envelope of a HGPS nucleus (D) shows a loss of peripheral heterochromatin and a thickening of the nuclear lamina compared with a normal nucleus (C). (C) Cytoplasm; (N) nucleus. Bars: A,B, 5 μM; C,D, 300 nm.

Figure 4.

Alterations in histone methylation patterns in HGPS fibroblasts. Normal and HGPS fibroblasts from female donors were double-labeled with antibodies against lamins A/C (red) and against trimethylation of either Lys 9 in histone H3 (H3K9me3; A,B), Lys 27 in histone H3 (H3K27me3; C,D), or Lys 20 in histone H4 (H4K20me3; E,F) (all green). Note the decrease of H3K9me3 (B) and H3K27me3 (D) and the increase of H4K20me3 (F) in the lobulated HGPS nuclei compared with normal nuclei (A,C,E). The decrease in H3K27me3 is best observed at the inactive X chromosome, which is normally enriched in this histone modification (see arrowhead in C). Bars, 10 μM.

Figure 5.

Schematic diagram depicting potential pathways in the epigenetic regulation of chromatin by lamins A/C. Red arrows indicate a direct regulation and/or interaction, whereas black arrows indicate indirect evidences for a role of lamins A/C in the regulation of the respective factors. The retinoblastoma protein may be an important link in these processes (for details, see the text).