Nuclear matrix-like filaments and fibrogranular complexes form through therearrangement of specific nuclear ribonucleoproteins

The behavior of nuclear pre-mRNA-binding proteins after their nuclease and/ or salt-induced release from RNA was investigated. After RNase digestion or salt extraction, two proteins that initially exist as tetramers (A2)(3)B1 in isolated heterogeneous nuclear ribonucleoprotein (hnRNP) complexes quant itatively reassociated to form regular helical filaments ranging in length from 100 nm to >10 mu m. In highly magnified preparations prepared for scan ning transmission electron microscopy, single filaments have diameters near 18 nm. In conventional negatively stained preparations viewed at low magni fication, the diameters of the thinnest filaments range from 7 to 10 nm. At protein concentrations of >0.1 mg/ml, the filaments rapidly aggregated to form thicker filamentous networks that look like the fibrogranular structur es termed the "nuclear matrix." Like the residual material seen in nuclear matrix preparations, the hnRNP filaments were insoluble in 2 M NaCl. Filame nt formation is associated with, and may be dependent on, disulfide bridge formation between the hnRNP proteins. The reducing agent 2-mercaptoethanol significantly attenuates filament assembly, and the residual material that forms is ultrastructurally distinct from the 7- to 10-nm fibers. In additio n to the protein rearrangement leading to filament formation, nearly one-th ird of the protein present in chromatin-clarified nuclear extracts was conv erted to salt-insoluble material within 1 min of digestion with RNase. Thes e observations are consistent with the possibility that the residual materi al termed the nuclear matrix may be enriched in, if not formed by, denature d proteins that function in pre-mRNA packaging, processing, and transport.