Identification of a widespread nuclear actin binding protein

Abstract

THE many different cellular functions so far shown to involve actin and to be regulated by specific actin binding proteins are located primarily, if not exclusively, in the cytoplasm^1–3. Actin is also found in the nucleus of various cells^4–7, but because of the problems of cell fractionation the significance of nuclear actin has remained unclear. The large amphibian oocyte nucleus (germinal vesicle), however, can be isolated manually with little cytoplasmic contamination. This nucleus contains high concentrations (4–6 mg ml⁻¹) of mostly soluble, although polymerization-competent β- and γ-actin^8–14, which exists in a nucleocytoplasmic exchange pool¹⁵. The findings that drastic effects on transcription and chromosome morphology are caused by the injection of actin antibodies or actin binding proteins into germinal vesicles^16,17, and that a factor required for accurate transcription by RNA polymerase II is actin¹⁸, suggest that nuclear actin is involved in specific nuclear functions. We have recently identified^19,20 two main components in Xenopus laevis oocytes with actin binding activities; one of these activities is Ca²⁺-dependent, is located predominantly, if not exclusively, in the cytoplasm and is attributable to gelsolin. Here we report that the second component, having a Ca²⁺-independent activity, is a heterodimeric acting binding protein; this protein is markedly enriched in the nuclei of oocytes and somatic cells of amphibia, but also occurs in nuclei of other vertebrate cells.