The non-histone proteins associated with eukaryotic nuclear chromatin are incompletely characterized and their function is poorly understood. Thirty years ago, the presence of actin in the eukaryotic nucleus was reported for first time. Since then, several biochemical studies have identified actin and myosin as significant constituents of isolated nuclear matrix from a variety of cells. The studies cited above and others make a strong case for presence of actin in nuclei, but do not implicate actin as a component of eukaryotic chromosomes.Our examination of cells associated with developing ovarian follicles of lepidopterans confirmed that under routine immunocytochemical conditions, no actin can be detected with anti-actin antibodies in the follicle cell nuclei (Fig. 1a) . However, Fig. 1b demonstrates that endonuclease pretreatment of the same preparation to remove DNA followed by anti-actin antibody staining uncovers the presence of nuclear actin. Moreover, by employing squash preparations of Drosophila salivary glands and the same endonuclease digestion, it is clear that the nuclear actin is directly associated with the polytene chromosomes (Fig. 2a,b).
Postmitotic expansion of cell nuclei requires nuclear actin filament bundling by α‐actinin 4
