Antibodies against distinct nuclear matrix proteins are characteristic for mixed connective tissue disease.

Specific nuclear proteins, separated according to their molecular weight (mol. wt) by polyacrylamide gel electrophoresis (PAGE) and subsequently transferred to nitrocellulose sheets, are able to bind antibodies in sera from patients suffering from different types of connective tissue diseases. Antibodies against a characteristic set of nuclear protein antigens are found in sera from patients with mixed connective tissue disease (MCTD). Screening of 21 MCTD sera revealed a typical immunoblot pattern with major protein antigens of mol. wt 70,000 (20/21) (not identical with the Scl-70 antigen characteristic for scleroderma), mol. wt 31,000 (17/21), two proteins around mol. wt 23,000 (15/21) and two around mol. wt 19,000 (10/21). The 70,000, 23,000 and 19,000 antigens appeared to be rather insoluble nuclear proteins (i.e. components of the nuclear matrix). On behalf of their structural character they were present in nuclei from several types of cells but only in low amounts detectable in salt extracts of thymus acetone powder. The presence of antibodies directed against the mol. wt 70,000 antigen correlated strongly with the diagnosis of MCTD. This 70,000 antigen is not identical with the RNP antigen, a soluble ribonuclease sensitive ribonucleoprotein, since antibodies against nuclear RNP can be separated from anti-nuclear matrix antibodies by affinity chromatography using immobilized thymus salt extract. The distinct character of soluble nuclear RNP and structural nuclear matrix antigens is further supported by the fact that from 14 other anti-RNP sera obtained from patients with systemic lupus erythematosus (SLE), only three contained antibodies against the mol. wt 70,000 protein. Since the immunoblot pattern obtained with MCTD sera mostly was clearly distinguishable from the patterns obtained with sera from patients with related connective tissue diseases our results suggest that the immunoblotting technique might be useful as a diagnostic tool and support the concept of MCTD as a distinct entity.

Anti-nuclear matrix antibodies in mixed connective tissue disease.

Purified serum antibodies of patients suffering from mixed connective tissue disease were tested for their immunological specificity against nuclear constituents of HeLa S3 cells. In the indirect immunofluorescent staining technique, using cells and nuclei as targets, a typical speckled intranuclear staining pattern was obtained, that persisted after degradation and extraction of all nucleic acids and their associated proteins. This treatment of nuclei with detergents, DNase, RNase and high salt concentrations leave intact only the so-called nuclear matrix which is an intranuclear proteinaceous network. Further proof that nuclear matrix proteins were targets of the autoimmune reaction was obtained after separation of these proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis and electrophoretic transfer to nitrocellulose (blotting). A specific number of blot-transferred matrix proteins reacted with purified serum antibodies of 10 patients with mixed connective tissue disease, whereas this reaction was negative with normal healthy individuals. IgG preparations of 7 patients with systemic lupus erythematosus showed a weak, if any, reaction with matrix constituents. Obviously, in some connective tissue diseases serum antibodies are expressed which are directed to specific nuclear matrix antigens.

Adenoviral heterogeneous nuclear RNA is associated with host cell proteins.

In this study irradiation of intact cells with 244-nm ultraviolet light was used to cross-link hnRNA to proteins that are closely associated with it. In this way proteins interacting specifically with hnRNA could be identified excluding the possibility of non-specific binding of proteins to the RNA during cell fractionation. In uninfected HeLa cells two polypeptides of 41500 and 43000 molecular weight can be cross-linked very efficiently to hnRNA. Other proteins, with molecular weights of 36000 and 37000, were covalently linked less effectively. Irradiation of adenovirus-infected cells results in the cross-linking of the same polypeptides to hnRNA. It was found, however, that hnRNA from adenovirus-infected cells contains both viral and host transcripts. Both classes of transcripts are quantitatively associated with the nuclear matrix and can be cross-linked by irradiation with equal efficiency. To determine whether both adenoviral and cellular transcripts are associated with the same proteins, the cross-linked adenoviral hnRNA-protein complexes were isolated by preparative hybridization to adenoviral DNA immobilized on Sepharose. The results show that the purified cross-linked adenoviral hnRNA-protein complexes also contain the host 41500-Mr and 43000-Mr proteins as major components. This suggests that in the infected cell adenoviral-specific hnRNA is associated with host proteins and that the structural organization of viral hnRNA-protein complexes in infected cells probably is similar to the organization of host hnRNA in uninfected cells.

hnRNA and its attachment to a nuclear protein matrix.

In this study, DNA-depleted nuclear protein matrices are isolated from HeLa S3 cells. These nuclear matrices consist of peripheral laminae, residual nucleoli, and internal fibrillar structures. High molecular weight, heterogeneous nuclear RNA (hnRNA) is quantitatively associated with these structures and can be released intact only by affecting the integrity of the matrices. It is, therefore, concluded that hnRNA is part of a highly organized nuclear structure. By irradiation of intact cells or isolated nuclear matrices with ultraviolet light, proteins tightly associated with hnRNA can be induced to cross-link with the RNA. Performing the cross-linking in vivo is an extra guarantee that only hnRNA-protein (hnRNP) complexes existing in the intact cell are covalently linked. Such hnRNP complexes were isolated and purified under conditions that completely dissociate nonspecific RNA-protein complexes. By comparison of the hnRNP found in nuclear matrices and the published data on the composition of hnRNP particles, it was found that the so-called hnRNP "packaging" proteins (32,000-38,000 mol wt) were not efficiently cross-linked to hnRNA by UV irradiation. They were, however, present in the matrix preparations, bound to hnRNA, because they were released from nuclear matrices after ribonuclease treatment of these structures. On the other hand, two major hnRNPs (41,500 and 43,000 mol wt) were efficiently cross-linked to hnRNA. These proteins were not released by ribonuclease treatment, which suggests that they are involved in the binding of hnRNA to the nuclear matrix.

Free cytoplasmic messenger ribonucleoprotein complexes from rabbit reticulocytes.

Free cytoplasmic globin mRNA containing mRNP-particles were isolated from rabbit reticulocytes by zonal sucrose gradient centrifugation and their properties were compared with mRNP particles isolated in the same way from EDTA-dissociated reticulocyte polyribosomes. The average poly(A)-length of 9S mRNA from free cytoplasmic mRNP was 17-20 nucleotides being about two times shorter than the average poly(A)-length of polysomal 9S mRNA. The protein composition of the free cytoplasmic mRNP particles disclosed the absence of the 76,000 dalton protein which is associated with the 3'poly(A)-segment of polysomal globin mRNA. It was concluded that free cytoplasmic mRNP-particles from rabbit reticulocytes can be classified as "old" mRNP in a post-translational phase. Free cytoplasmic mRNPs were translated in heterologous cell-free systems as well as in Xenopus laevis oocytes. Addition of hemin stimulated the synthesis of alpha-globin in all systems, while the presence of the cap analogue m7G(5')p inhibited translation of free cytoplasmic mRNA completely. The latter finding suggested that free cytoplasmic mRNA has a 5' terminal "cap". Shortening of the poly(A)-segment with concomitant loss of the 76,000 dalton protein may lead to less efficient translation of free cytoplasmic mRNP.