Binding of the androgen receptor to the nuclear matrix of human foreskin

Isoelectric focusing and 2D electrophoresis of the human androgen receptor

Nuclear androgen receptors from cultured genital skin fibroblasts were analyzed by non-denaturing isoelectric focusing (IEF) in ultrathin polyacrylamide gels before and after photoaffinity labeling with [3H]methyltrienolone. Both reversibly and covalently labeled receptors focused at pH 5.28 +/- 0.20 when extracted from nuclei with high salt. Lowering of the salt concentration yielded, in both cases, a second species which focused at pH 7.16. This species became predominant when nuclei were sonicated in IEF sample buffer containing no salt, even after extensive nucleic acid digestion. Low salt cytosols from both prostate and foreskin focused as a single peak of pI: 4.93 +/- 0.31 which remained unchanged when KCl was added to the cytosol up to a concentration of 0.6 M. SDS-polyacrylamide gel electrophoresis of photoaffinity labeled receptors revealed labeled proteins with Mw 90-95 kDa. Two-dimensional electrophoresis of photoaffinity labeled nuclear receptors, extracted in low or high salt, showed that the two isoforms (pI 5.28 and 7.16) contain the same steroid-binding subunit with Mw 90-95 kDa. Nuclear receptors from 4 patients with the receptor positive form of the Complete Androgen Insensitivity Syndrome (CAIS, Rc+) were analyzed by non-denaturing IEF: a single species was observed, focusing at pH 6.0 whether in high or low salt conditions. These results indicate that the nuclear androgen receptor is an acidic protein with pI 5.28 and Mw 90-95 kDa under maximum protein dissociation conditions. When extracted under low salt conditions, it can be isolated in a neutral form (pI 7.16) suggesting its association with a nuclear protein. Receptors of (CAIS, Rc+) patients have an abnormal charge and show no pI shift upon lowering of the salt concentration suggesting that this shift could be a significant step in the mechanism of action of androgens.

Nuclear structure and the three-dimensional organization of DNA

The organization of DNA within the nucleus has been demonstrated to be both cell and tissue specific and is arranged in a non-random fashion in both sperm and somatic cells. Nuclear structure has a pivotal role in this three-dimensional organization of DNA and RNA and contributes as well to forming fixed organizing sites for nuclear functions, such as DNA replication, transcription, and RNA processing. In sperm, DNA is also organized in a specific fashion by the nuclear matrix and DNA-protamine interactions. Within somatic cells, the nuclear matrix provides a three-dimensional framework for the tissue specific regulation of genes by directed interaction with transcriptional activators. This differential organization of the DNA by the nuclear matrix, in a tissue specific manner, contributes to tissue specific gene expression. The nuclear matrix is the first link from the DNA to the entire tissue matrix system and provides a direct structural linkage to the cytomatrix and extracellular matrix. In summary, the tissue matrix serves as a dynamic structural framework for the cell which interacts to organize and process spatial and temporal information to coordinate cellular functions and gene expression. The tissue matrix provides a structural system for integrating form and function.