Minor lamin polypeptides from rat liver nuclei can be cross-linked into heteropolymers by disulfide bridges

Phorbol esters induce transient changes in the accessibility of the carboxy-terminal domain of nuclear lamin A

Treatment of human epithelial cells in culture with phorbol esters (TPA) gives rise to a transient and reversible loss of accessibility to antibodies of the nonhelical carboxy-terminal domain of nuclear lamin A that distinguishes it from lamin C. No change in the accessibility of epitopes present in the common domain of lamins A and C was observed. Loss of accessibility of lamin A was not due to proteolytic degradation nor to modification of the isoelectric point of lamin A and did not depend upon protein kinase C activation nor protein synthesis. Perturbation of desmosome organization by growth in low calcium blocked the effect of TPA on lamin A. Prolonged exposure to nocodazole, one of the effects of which is a perinuclear collapse of intermediate filaments, also blocked the effect of TPA on lamin A. These results suggest that the initial target of TPA may be at the level of cell-cell contacts and that the perturbation induced by TPA may be propagated via the structural link formed by intermediate filaments between the cell surface and the nucleus, giving rise to a change in conformation of the carboxy-terminal domain of lamin A or to an interaction of this domain with another nuclear component. These results form the basis for the hypothesis that the interphase nuclear lamina may play an active role in the process of mechanochemical signal transduction.

Autoantibodies to major and minor nuclear lamins are not restricted to autoimmune diseases

Autoantibodies to lamins, the major polypeptide components of the nuclear lamina, have been reported in selected sera from patients with autoimmune diseases, including anti-lamin B in systemic lupus erythematosus (SLE) and anti-lamins AC in autoimmune chronic active hepatitis (CAH). We have studied the frequency, specificity, and isotypy of autoantibodies to major and minor lamins by immunoblotting on purified rat liver lamins in 190 sera from normal controls (n = 62), rheumatic disease controls (n = 42), and autoimmune disease patients (n = 86). The frequency of anti-lamin in normal controls was 85.5%, and ranged from 77 to 100% in the other groups. Anti-lamin frequency was not related to age, sex, or disease duration. Reactivity with lamin A or with minor lamins only was observed with 7 various sera and 2 normal sera, respectively. Between groups, the proportions of reactive sera were not different for lamins AC (18-47%) and for lamin B (22-36%). In particular, anti-lamin B and anti-lamins AC were not more common in SLE or CAH than in normal sera. The most frequent lamin specificity of SLE sera was anti-lamins ABC. Anti-lamin isotypes were IgG and/or IgM. Titers of IgM antibodies were not higher in any group. However, IgG anti-lamin titers were higher in CAH than in normal, ankylosing spondylitis, or SLE sera. The highest end point titers (greater than or equal to 1:3200) were observed with CAH, SLE, and rheumatoid arthritis (RA) sera with IgG anti-lamins AC, B, or ABC, or with IgM anti-lamins ABC. None of these SLE and RA patients had evidence of liver disease. Reactivity with minor lamins was more frequent in CAH. We conclude that anti-lamin autoantibodies are present in sera from most individuals and that the highest titers are found in sera from patients with autoimmune diseases.

Differential accessibility of the tail domain of nuclear lamin A in interphase and mitotic cells

Human autoantibodies reactive against the tail domain exclusive to lamin A and absent from lamin C have been used for immunofluorescence studies on human fibroblast and epithelial cells. These autoantibodies were seen to react on mitotic cells where lamin A is present in a soluble depolymerized form and to react against lamin A in assembled interphase nuclear lamina after in situ extraction of chromatin. Taken together, these results support the suggestion that the tail domain of lamin A may be involved in the putative interaction of lamin A with chromatin.

In vitro posttranslational modification of lamin B cloned from a human T-cell line

Autoimmune diseases are characterized by spontaneously occurring autoantibodies which have proven to be useful reagents for the characterization of specific nuclear proteins. Using a monoclonal autoantibody (72B9) derived from a murine lupus strain, we have cloned a cDNA from the human T-cell line MOLT-4, which encodes nuclear lamin B. The identity of the encoded protein as lamin B was established by both biochemical and immunological criteria. Inspection of the deduced amino acid sequence of lamin B revealed the presence in coil 1B of the alpha-helical domain of a leucine heptad repeat region. Analysis of mRNA in HL60 and MOLT-4 cells, which express only lamin B, or HeLa cells, which express all three major lamins (A, B, and C), together with the comigration of in vitro-translated product with isolated HeLa cell lamin B by two-dimensional gel electrophoresis, suggests that a single lamin B is expressed in mammalian somatic cells. In vitro translation with the cDNA clone revealed an EDTA-sensitive posttranslational modification which resulted in an increase in the apparent molecular weight to that equivalent to the native in vivo-synthesized lamin B protein. This in vitro modification included incorporation of a product of mevalonolactone and required an intact carboxy terminus.

In vitro posttranslational modification of lamin B cloned from a human T-cell line

Autoimmune diseases are characterized by spontaneously occurring autoantibodies which have proven to be useful reagents for the characterization of specific nuclear proteins. Using a monoclonal autoantibody (72B9) derived from a murine lupus strain, we have cloned a cDNA from the human T-cell line MOLT-4, which encodes nuclear lamin B. The identity of the encoded protein as lamin B was established by both biochemical and immunological criteria. Inspection of the deduced amino acid sequence of lamin B revealed the presence in coil 1B of the alpha-helical domain of a leucine heptad repeat region. Analysis of mRNA in HL60 and MOLT-4 cells, which express only lamin B, or HeLa cells, which express all three major lamins (A, B, and C), together with the comigration of in vitro-translated product with isolated HeLa cell lamin B by two-dimensional gel electrophoresis, suggests that a single lamin B is expressed in mammalian somatic cells. In vitro translation with the cDNA clone revealed an EDTA-sensitive posttranslational modification which resulted in an increase in the apparent molecular weight to that equivalent to the native in vivo-synthesized lamin B protein. This in vitro modification included incorporation of a product of mevalonolactone and required an intact carboxy terminus.

Transfection of human lamins A and C into mouse embryonal carcinoma cells possessing only lamin B

The peripheral lamina of eukaryotic nuclei is composed of polypeptides called lamins that vary in number from one to four according to organism, cell type, and differentiated state of the cells. Early embryonic cells and stem cells of mammals generally possess only lamin B while lamins A and C appear later during differentiation. To study the role of the late appearance of lamins A and C in the differentiated phenotype, we have performed transfection of cDNAs coding for human lamins A or C into mouse embryonal carcinoma (EC) cell lines F9 and P19 lacking these two lamins. Transient transfections have shown that lamins A or C could be expressed, translocated to the peripheral lamina, and distributed into daughter cell nuclei after mitosis. These results demonstrated that EC cells devoid of lamins A and C nevertheless possessed the appropriate mechanisms for the localization and mitotic redistribution of exogenous lamins A and C.