Actomyosin proteins in the nuclear matrix: Implications for structure

Actin has been found as a component of the nucleus and has recently been implicated in the organization of chromatin remodeling complexes. We have investigated the presence of potential actomyosin regulatory proteins that might be present in the nucleus and whether they were associated with the nuclear matrix and nuclear swelling. Using proteomics we identified two unique proteins that may be involved in an actomyosin system within the nucleus. The first is NM-18, a new form similar to myosin regulatory light chain with the unique ability to bind to a specific monoclonal antibody against α-sarcomeric actin. We also identified the actin binding protein, transgelin (NPB-2) within the nucleus. Both regulatory proteins were present only in the nuclear matrix fraction and were found to be expressed at lower levels in human prostate cancer tissue compared to adjacent normal prostate tissue through proteomic studies. Nuclear morphometry has been shown to dramatically change with the development of prostate cancer, and this is associated with the changes in these actomyosin regulatory proteins. We also examined nuclear structure by swelling isolated, demembranated nuclei in vitro to nine times their volume with specific sulfated compounds and then recontracting them back to their normal size. This swelling is dependent on DNA, and with the removal of DNA, the nucleus is elastic. We propose that the higher order organization of DNA loops to the nuclear matrix could explain this phenomenon. Nuclear lamins do not define the border of the expanded nuclei, but reorganize centrally. DNA masked the availability of the NM-18 protein to antibodies, but after swelling was revealed to be present throughout the expanded and recontracted nucleus. These results have potential implication in further revealing dynamic aspects of nuclear structure and their changes in cancer. The role of actomyosin regulating proteins in cancer has been studied primarily in fibroblasts and cultured cells. Here they are compared in prostate epithelial cells in fresh samples from the same patient derived from adjacent areas of benign and malignant growth.