The B1C8 protein is in the dense assemblies of the nuclear matrix and relocates to the spindle and pericentriolar filaments at mitosis

Dynamic association of proteins with the pre-mRNA branch region

The association of proteins with the branch site region during pre-mRNA splicing was probed using a novel methodology to site-specifically modify the pre-mRNA with the photo-reagent benzophenone. Three sets of proteins were distinguished by the kinetics of their associations with pre-mRNAs, by their association with discrete splicing complexes, and by their differing factor requirements. An early U1 snRNP-dependent cross-link of the branch region to a p80 species was followed by cross-links to p14, p35, and p150 polypeptides associated with the U2 snRNP-pre-mRNA complex. Concomitant with formation of the spliceosome, a rearrangement of protein factors about the branch region occurred, in which the p35 and p150 cross-links were replaced by p220 and p70 species. These results establish that the branch region is recognized in a dynamic fashion by multiple distinct proteins during the course of spliceosomal assembly.

Association of nuclear matrix antigens with exon-containing splicing complexes

mAbs raised against the human nuclear matrix (anti-NM)1 mAbs have been used to investigate the role of nuclear matrix antigens in pre-mRNA processing. The three anti-NM mAbs used in this study recognize antigens that are highly localized to nuclear matrix speckles. Surprisingly, all three of these mAbs preferentially immunoprecipitate splicing complexes containing exon sequences. The anti-NM mAbs efficiently immunoprecipitate the exon product complex but not complexes containing the lariat product after the second step of splicing. Two of the anti-NM mAbs completely inhibit pre-mRNA splicing in vitro. However, none of the anti-NM mAbs appear to recognize factors stably associated with splicing snRNPs. The three anti-NM mAbs predominantly react with distinct high molecular weight antigens, which belong to a class of nuclear proteins that selectively precipitate with Ser-Arg protein-splicing factors in the presence of high Mg2+ concentrations. Immunological, biochemical, and cell biological data indicate that two of the NM antigens are related to the defined set of Ser-Arg proteins. The results suggest the existence of an extended Ser-Arg family as a component of the nuclear matrix.

The amino-terminal region of the retinoblastoma gene product binds a novel nuclear matrix protein that co-localizes to centers for RNA processing

The tumor suppressing capacity of the retinoblastoma protein (p110RB) is dependent on interactions made with cellular proteins through its carboxy-terminal domains. How the p110RB amino-terminal region contributes to this activity is unclear, though evidence now indicates it is important for both growth suppression and regulation of the full-length protein. We have used the yeast two-hybrid system to screen for cellular proteins which bind to the first 300 amino acids of p110RB. The only gene isolated from this screen encodes a novel 84-kD nuclear matrix protein that localizes to subnuclear regions associated with RNA processing. This protein, p84, requires a structurally defined domain in the amino terminus of p110RB for binding. Furthermore, both in vivo and in vitro experiments demonstrate that p84 binds preferentially to the functionally active, hypophosphorylated form of p110RB. Thus, the amino terminus of p110RB may function in part to facilitate the binding of growth promoting factors at subnuclear regions actively involved in RNA metabolism.

Nuclear architecture supports integration of physiological regulatory signals for transcription of cell growth and tissue-specific genes during osteoblast differentiation

During the past several years it has become increasingly evident that the three-dimensional organization of the nucleus plays a critical role in transcriptional control. The principal theme of this prospect will be the contribution of nuclear structure to the regulation of gene expression as functionally related to development and maintenance of the osteoblast phenotype during establishment of bone tissue-like organization. The contributions of nuclear structure as it regulates and is regulated by the progressive developmental expression of cell growth and bone cell related genes will be examined. We will consider signalling mechanisms that integrate the complex and interdependent responsiveness to physiological mediators of osteoblast proliferation and differentiation. The focus will be on the involvement of the nuclear matrix, chromatin structure, and nucleosome organization in transcriptional control of cell growth and bone cell related genes. Findings are presented which are consistent with involvement of nuclear structure in gene regulatory mechanisms which support osteoblast differentiation by addressing four principal questions: 1) Does the representation of nuclear matrix proteins reflect the developmental stage-specific requirements for modifications in transcription during osteoblast differentiation? 2) Are developmental stage-specific transcription factors components of nuclear matrix proteins? 3) Can the nuclear matrix facilitate interrelationships between physiological regulatory signals that control transcription and the integration of activities of multiple promoter regulatory elements? 4) Are alterations in gene expression and cell phenotypic properties in transformed osteoblasts and osteosarcoma cells reflected by modifications in nuclear matrix proteins? There is a striking representation of nuclear matrix proteins unique to cells, tissues as well as developmental stages of differentiation, and tissue organization. Together with selective association of regulatory molecules with the nuclear matrix in a growth and differentiation-specific manner, there is a potential for application of nuclear matrix proteins in tumor diagnosis, assessment of tumor progression, and prognosis of therapies where properties of the transformed state of cells is modified. It is realistic to consider the utilization of nuclear matrix proteins for targeting regions of cell nuclei and specific genomic domains on the basis of developmental phenotypic properties or tissue pathology.