Lamin A is not synthesized as a larger precursor polypeptide

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.

Isoprenylation is required for the processing of the lamin A precursor

The nuclear lamina proteins, prelamin A, lamin B, and a 70-kD lamina-associated protein, are posttranslationally modified by a metabolite derived from mevalonate. This modification can be inhibited by treatment with (3-R,S)-3-fluoromevalonate, demonstrating that it is isoprenoid in nature. We have examined the association between isoprenoid metabolism and processing of the lamin A precursor in human and hamster cells. Inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase by mevinolin (lovastatin) specifically depletes endogenous isoprenoid pools and inhibits the conversion of prelamin A to lamin A. Prelamin A processing is also blocked by mevalonate starvation of Mev-1, a CHO cell line auxotrophic for mevalonate. Moreover, inhibition of prelamin A processing by mevinolin treatment is rapidly reversed by the addition of exogenous mevalonate. Processing of prelamin A is, therefore, dependent on isoprenoid metabolism. Analysis of the conversion of prelamin A to lamin A by two independent methods, immunoprecipitation and two-dimensional nonequilibrium pH gel electrophoresis, demonstrates that a precursor-product relationship exists between prelamin A and lamin A. Analysis of R,S-[5-3H(N)]mevalonate-labeled cells shows that the rate of turnover of the isoprenoid group from prelamin A is comparable to the rate of conversion of prelamin A to lamin A. These results suggest that during the proteolytic maturation of prelamin A, the isoprenylated moiety is lost. A significant difference between prelamin A processing, and that of p21ras and the B-type lamins that undergo isoprenylation-dependent proteolytic maturation, is that the mature form of lamin A is no longer isoprenylated.

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.

A major 62-kD intranuclear matrix polypeptide is a component of metaphase chromosomes

We have isolated and partially characterized a major intranuclear matrix polypeptide from rat liver. This polypeptide, which is reversibly stabilized into the intranuclear matrix under conditions which promote intermolecular disulfide bond formation, has a Mr of 62,000 and pI of 6.8-7.2 as determined by two-dimensional IEF/SDS-PAGE. A chicken polyclonal antiserum was raised against the polypeptide purified from two-dimensional polyacrylamide gels. Affinity-purified anti-62-kD IgG was prepared and used to immunolocalize this polypeptide in rat liver tissue hepatocytes. In interphase hepatocytes the 62-kD antigen is localized in small, discrete patches within the nucleus consistent with the distribution of chromatin. The staining is most prominent at the nuclear periphery and somewhat less dense in the nuclear interior. Nucleoli and cytoplasm are devoid of staining. During mitosis the 62-kD antigen localizes to the condensed chromosomes with no apparent staining of cytoplasmic areas. The chromosomal staining during mitosis is uniform with no suggestion of the patching seen in interphase nuclei. Fractionation and immunoblotting studies using rat hepatoma tissue culture cells blocked in metaphase with colcemid confirm the chromosomal localization of this 62-kD intranuclear protein during mitosis. The 62-kD polypeptide fractionates completely with metaphase chromosome scaffolds generated by sequential treatment of isolated chromosomes with DNAse I and 1.6 M NaCl, suggesting that this major 62-kD intranuclear protein may be involved in maintaining metaphase chromosomal architecture.